
Well, I'll lump all these 3D social worlds under "The Sims Online", because once you remove the "OMG, you could make real money out of it" BS out of SL, that's sorta what you get.
Now let's compare them to other online worlds.
1. MMOs. IMHO they aren't just successful because people are playing a game for score. Well, ok, indirectly it is. The thing is, the "game" aspect influences the social aspect too. It gives everyone a common topic to talk to, and a reason to talk to perfect strangers.
It's, IMHO, a bit like going at a, say, dog owners' club or a linux users' group. You can always start a talk about dogs or respectively linux. You have a common topic right there.
Further more, the game aspect gives you a reason or excuse to organize groups, have common events, have some chat channels you can join, etc.
Even IRC is sorted into channels based on some topic. So if you know your interests are, say, WoW and cats, you can look for a channel having either for a topic.
By comparison, think of being dumped in a foreign city where you don't know anyone and have no idea what anyone is interested in, or why would they even want to talk to you. Chances are none of us would even try talking to random people on the street. We'll just stick to doing what we came there for, and bugger off as soon as we're done with that.
Well, big surprise that people buggered off from TSO and the like.
And I'll say that it's not very surprising that the only ones happy to stay in such a foreign town where nobody knows them, is those looking for that kind of anonymity and chance to be someone else. E.g., those looking for sex, antisocial fuckwits, and a few other categories.
2. MUDs and IRC.
While both overlap the previous point a bit, they have another thing going for them: they've got a superb chat interface.
Think about it. All the 3D interface and 3D world to navigate, and whatnot, only get in the way when you just want to chat. If I want to chat to someone, I want to just start typing and there better be as little as possible that gets in the way, or steals screen space from the chat. Basically my first choice would be some kind of IM (I'm open to pretty much any of them), followed by a clean chat interface like IRC or a MUD.
Games like WoW have the saving grace that there is actually a game that uses that 3D interface. It's one good excuse to put up with it. In a 3D world that doesn't actually have a game, that excuse just vanishes. Why do I need all that extra baggage and why should I learn extra skills, if I can have that social chat without them just the same? In fact, even better.