Mystery?

Alright, so I'm talking on AIM and for months people have been on mobile when not "available". Anyone who uses AIM knows what I'm talking about. So tonight I'm watching as everyone who was mobile starts simply going offline. All of them. Then when two are online, one goes to bed and logs off. He has been mobile since I he got on my list. Then was shown as logged off. Anyone know what is going on here?