I'd like to make a few points in defence of SL...
1) It's not a game, it's an environment for building/scripting and designing and performing for a worldwide audience, which also happens to allow some residents to make money and also, yes, socialise/chat.
2) Socialising in SL is no different to having a chat via IRC, or a yahoo chat room, and I'll assume, that given that most of us have a background in IT, we've all done one or the other, or even both, at some point in our lifetimes.
3) Yes, some people actually do use it to socialise in virtual sense, rather than in their real life, because some people have a disability that stops them from having a "life" in the real world. You'd be surprised how much power they can regain from having the freedom and control that they don't have in their real lives.
3) er, yeah, I can't really think of anything else, but there is more to it than just sad people having cybersex. Ignorance