Update:

Steam is fixing the error at the moment. Your friends list should be returning soon.

This isn’t an actual post, so much as a notice. If you’ve been on Steam recently, you may notice that your friends list has been completely wiped out for some.

This doesn’t apply to you specifically; this is a problem across the entire Steam community.

At the moment, there hasn’t been a fix, but apparently Steam is working to fix the problem.

http://forums.steampowered.com/forums/showthread.php?t=1156816

http://forums.steampowered.com/forums/showthread.php?t=1156897

http://forums.steampowered.com/forums/showthread.php?t=1156864