What is it exactly that you’re looking to do? That might help us direct you to a place that’s appropriate or a good fit for you.
Social networking sites are all the rage so there’s no lack of them, and some sites that are more subject-driven are also out there. Personally I’m not fond of MySpace at all, I’ve visited a few pages that had so much going on in the way of background that I couldn’t focus on the materials being presented.
While Facebook has a overwhelming amount of useless but fun applications the interface itself is fairly clean. It’s made staying in touch with people that I already know and hooking up with people who share similar interests far more interesting.
In short, the childish shit is generally an option and not a requirement. It may take awhile for you to do anything more than test the waters with each social networking possibility you consider.