Sometimes my steam friends list will list many of my friends as online, and allow me to send messages, although my friends will not receive these messages. I think this often happens after a momentary lapse in my connection to steam, then says all my friends that were online beforehand still appear to be online. However, if I set my online state to offline then reconnect, all freinds are listed as offline no connection is made, and Steam must be restarted to reconnect.