It's an issue with Steam and fglrx. Perform a sudo apt-get remove fglrx-driver, install Steam, login, and reinstall fglrx-driver. Steam will work if you install it using the open source driver (xorg-xserver-video-radeon) first, then install fglrx.

Oh thanks very much! :) To be honest it doesn't surprise me that my graphics driver was involved, it's been giving loads of other issues too lol. AMD support for Linux seems terrible. Thanks again :)

I have same problem on Xubuntu 14.04 AMD64. I just installed Steam from Ubuntu Software Centre. Trying to start it and that's the result:

$ steam
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(0_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20140524130659_1.dmp
/home/pioruns1/.local/share/Steam/steam.sh: line 755: 7290 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
mv: cannot stat ‘/home/pioruns1/.steam/registry.vdf’: No such file or directory
Installing bootstrap /home/pioruns1/.local/share/Steam/bootstrap.tar.xz
Reset complete!
Restarting Steam by request...
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME has been set by the user to: /home/pioruns1/.local/share/Steam/ubuntu12_32/steam-runtime
Installing breakpad exception handler for appid(steam)/version(0_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20140524130702_1.dmp
/home/pioruns1/.local/share/Steam/steam.sh: line 755: 7417 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-8bbacadd-3a79-4cfc-87c5-d31412140524
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-069cbc92-6d75-4e91-a90e-be6b72140524

Drivers:
$ dpkg -l |grep fgl
ii fglrx-amdcccle-updates 2:13.350.1-0ubuntu2 amd64 Catalyst Control Center for the AMD graphics accelerators
ii fglrx-updates 2:13.350.1-0ubuntu2 amd64 Video driver for the AMD graphics accelerators
ii fglrx-updates-dev 2:13.350.1-0ubuntu2 amd64 Video driver for the AMD graphics accelerators (devel files)

System:
$ uname -a
Linux pioruns1-desktop 3.13.0-24-generic #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04 LTS
Release: 14.04
Codename: trusty

Update: I downgraded fglrx drivers from fglrx-updates to standard fglrx:
$ dpkg -l |grep fgl
ii fglrx 2:13.350.1-0ubuntu2 amd64 Video driver for the AMD graphics accelerators
ii fglrx-amdcccle 2:13.350.1-0ubuntu2 amd64 Catalyst Control Center for the AMD graphics accelerators

Result:
steam
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(0_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20140524132041_1.dmp
/home/pioruns1/.local/share/Steam/steam.sh: line 755: 3525 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
mv: cannot stat ‘/home/pioruns1/.steam/registry.vdf’: No such file or directory
Installing bootstrap /home/pioruns1/.local/share/Steam/bootstrap.tar.xz
Reset complete!
Restarting Steam by request...
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME has been set by the user to: /home/pioruns1/.local/share/Steam/ubuntu12_32/steam-runtime
Installing breakpad exception handler for appid(steam)/version(0_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/crash_20140524132043_1.dmp
/home/pioruns1/.local/share/Steam/steam.sh: line 755: 3651 Segmentation fault (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-d88bb94e-97cb-4065-b993-6f2722140524
Finished uploading minidump (out-of-process): success = no
error: Couldn't connect to server

I installed standard open source driver from xorg. Result: steam is just starting up with some errors:
$ steam
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(0_client)
libGL error: dlopen /usr/lib/fglrx/dri/swrast_dri.so failed (/usr/lib/fglrx/dri/swrast_dri.so: cannot open shared object file: No such file or directory)
libGL error: dlopen /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so failed (/usr/lib/x86_64-linux-gnu/dri/swrast_dri.so: wrong ELF class: ELFCLASS64)
libGL error: dlopen /usr/lib/dri/swrast_dri.so failed (/usr/lib/dri/swrast_dri.so: cannot open shared object file: No such file or directory)
libGL error: dlopen /usr/lib32/fglrx/dri/swrast_dri.so failed (/usr/lib32/fglrx/dri/swrast_dri.so: cannot open shared object file: No such file or directory)
libGL error: failed to load driver: swrast
Installing breakpad exception handler for appid(steam)/version(1.0_client)
Installing breakpad exception handler for appid(steam)/version(1.0_client)
Installing breakpad exception handler for appid(steam)/version(1.0_client)
ILocalize::AddFile() failed to load file "public/steambootstrapper_english.txt".
[2014-05-24 13:24:53] Startup - updater built Feb 10 2014 16:03:16
[2014-05-24 13:24:53] Verifying installation...
[2014-05-24 13:24:53] Unable to read and verify install manifest /home/pioruns1/.local/share/Steam/package/steam_client_ubuntu12.installed
[2014-05-24 13:24:53] Verification complete
[2014-05-24 13:24:53] Downloading Update...
[2014-05-24 13:24:53] Checking for available update...
[2014-05-24 13:24:54] Package file tenfoot_misc_all.zip.b2edde04f992497dc6866f65bb881b84cce49c89 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_dicts_all.zip.c31ed9aaeb4a023fcbc8a26a4133ef8a6e9aae29 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_fonts_all.zip.7c5db178f5ebe8b1bb02aca53939040358a56342 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_ambientsounds_all.zip.888b796633f8a81789aeb026bc1aa3292d04f989 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_sounds_all.zip.vz.513680104410158b10b2b94d4ef23c83e17b7c63_1224523 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_images_all.zip.vz.f52fc4d51801f84405c6fb480038c85531af92a0_9151299 missing or incorrect size
[2014-05-24 13:24:54] Package file tenfoot_all.zip.vz.3b147243235ba685090eedd9aa52f6dba097f06b_837049 missing or incorrect size
[2014-05-24 13:24:54] Package file resources_misc_all.zip.vz.ac4ad0df6d6c67597d1430c34579dd22579ba897_3651556 missing or incorrect size
[2014-05-24 13:24:54] Package file resources_all.zip.vz.fa56e61397fa5b04872a19aca83e4da4d77f1e12_1026882 missing or incorrect size
[2014-05-24 13:24:54] Package file strings_en_all.zip.0e46d0876f14bf57359d272db416cdecb7edc1ae missing or incorrect size
[2014-05-24 13:24:54] Package file strings_all.zip.vz.8af20b3f86622b49261a52c06084d46af35af133_1727287 missing or incorrect size
[2014-05-24 13:24:54] Package file remoteui_all.zip.vz.9ffbcb2095d8a5bd8d63ec668e25431d0645c53a_143117 missing or incorrect size
[2014-05-24 13:24:54] Package file public_all.zip.vz.271f7734499837ceebf75f24ed52a00f046aba01_599537 missing or incorrect size
[2014-05-24 13:24:54] Package file bins_ubuntu12.zip.vz.1e8e9fb4546d311a808229cd85ff0167df33db40_21733339 missing or incorrect size
[2014-05-24 13:24:54] Package file bins_codecs_ubuntu12.zip.51512e1a8db77f08589d722bc51658f12df86d85 missing or incorrect size
[2014-05-24 13:24:54] Package file bins_misc_ubuntu12.zip.15f96223e8eca7303f8c0d608e1f897770bae140 missing or incorrect size
[2014-05-24 13:24:54] Package file webkit_ubuntu12.zip.vz.c55bd35c9ad3c20c6d9dc702e9d4e24e860cae92_12610322 missing or incorrect size
[2014-05-24 13:24:54] Package file miles_ubuntu12.zip.vz.1bc54dcef69759a8e789b0f71b516aedc1ceaa47_298518 missing or incorrect size
[2014-05-24 13:24:54] Package file sdl2_ubuntu12.zip.vz.b00480638e33c6f178a531ef09ec922809450289_273080 missing or incorrect size
[2014-05-24 13:24:54] Package file steam_ubuntu12.zip.vz.5447012941f19720c302fca304cf48517bfcafbc_873733 missing or incorrect size
[2014-05-24 13:24:54] Package file runtime_part0_ubuntu12.zip.8dbceaa0a4df25382f2b47e4bfefcc04db722275 missing or incorrect size
[2014-05-24 13:24:54] Package file runtime_part1_ubuntu12.zip.e700b4b9fc11337f8c9a16685fe06cd629796fd8 missing or incorrect size
[2014-05-24 13:24:54] Downloading update (0 of 191,475 KB)...
[2014-05-24 13:24:54] Downloading update (111 of 191,475 KB)...

Steam has started properly, but I am getting an error:
OpenGL GLX context is not using direct rendering, which may cause performance problems.

For more information visit https://support.steampowered.com/kb_article.php?ref=9938-EYZB-7457.

same issue [ radeon driver ]

mathieu@Miyauchi-QC-PC:~$ dpkg -l | grep fgl
mathieu@Miyauchi-QC-PC:~$ steam
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1401381906_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/assert_20140707113415_1.dmp
/home/mathieu/.steam/steam.sh: line 755: 18897 Segmentation fault      (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
mv: cannot stat ‘/home/mathieu/.steam/registry.vdf’: No such file or directory
Installing bootstrap /home/mathieu/.steam/bootstrap.tar.xz
Reset complete!
Restarting Steam by request...
Running Steam on ubuntu 14.04 64-bit
STEAM_RUNTIME has been set by the user to: /home/mathieu/.steam/ubuntu12_32/steam-runtime
Installing breakpad exception handler for appid(steam)/version(1401381906_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/assert_20140707113420_1.dmp
/home/mathieu/.steam/steam.sh: line 755: 19029 Segmentation fault      (core dumped) $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-b28d7422-3ae9-4da2-b558-9c2e62140707
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-095bbe9b-9bf3-4d86-a17f-805702140707
mathieu@Miyauchi-QC-PC:~$ 

small update. this looks to be caused by radeon driver not be able to load GL extensions...
running glxinfo only shows an error.

updated steam client now shows GL missing error instead of segfault

something is up with at least Kubuntu 14.04 and radeon driver... I am going to buy a new nvidia card