Ultimately, it's a toss-up between and FriendFeed.
If Flock's People sidebar were available as a separate, stand-alone download (à la the popular Facebook and Twitter tool, ), it would show all kinds of potential. In contrast, FriendFeed is awesome as is, but its usefulness is determined by breadth of adoption (or lack thereof), and convincing your friends that it's time to sign up for yet another social portal is easier said than done.