The same thing happened to me, i just logged on and i have no friends, when i logged off i had a full friends list. I called for support on it and it says that the servers are down. I don't know if that has anything to do with it, but its retarded that this has happened.
I just got online and my list is gone as well. The only thing I have used my account for is hulu plus, Tomb Raider, and Black Ops 2....are any of those a possible culprit?