Well, I've spent the last 15-20 minutes looking across Avatars United, and so far it seems little more than MySpace for virtual world characters.
Still, that didn't put me off from signing up! There seems to be a frighteningly lack of security or verification though. I can't see where it wouldn't have allowed me to sign up as Chaffro Linden if I had wanted to (in retrospect, that might not have been a good idea, but still). I understand that Linden Lab have given this site some blessing - either by buying it, or just acknowledging it, I'll investigate further - but I'm becoming a bit of a sucker for social networking, despite the problems it can often bring. I've actually made many new genuine friendships from Facebook, MySpace, Twitter and SL. Not just 'online acquaintances' either, I've spent time with these people in real life and continue to maintain those connections.
If you do sign up, as me as a friend! This is me!
EDIT: OK, it turns out that the Lab have acquired Enemy Within, the development company behind Avatars United; hence the massive upsurge of interest over the past few days.
It's a strange beast. To be honest, there seems as though there's little to actually explore within the site. Presumably it's meant to serve as a virtual world info-hub; I can import RSS feeds of blogs I follow (which, if they weren't virtual world related, would be pretty pointless). Like Facebook, you can send gifts to your friends and contacts, but they're going to cost you 'coins'. It also costs you coins to upload a custom background; if you don't want to invest any real-world money into the site, you're stuck with a choice of three or four. The 'upside' is, is that if you choose to have your background public, you can sell it (but only reaping back a lowly 7 coins per purchase, and there might be a sales limit per background).
Hopefully the Lab will start introducing some cross-network potential; and I'm hoping someone will come up with a Twitter-functional application, to allow shouts to appear on the Twitter timeline or Tweets to appear in your shout box. That could be interesting.
I wonder how long M Linden would keep people on his friends list then!