As far as social networks go, I like LinkedIn a lot. Nice site, lots of features, generally quite useful. In fact, I like it enough to be careful about who I link to ... for example, I only link to people I've met in person (or, at least, had a phone conversation with).
For some people, social networks are a game and the score is the number of connections. Thus, I get a fair number of "cold" link requests.
Here's a trick: if you want someone like me to link to you, you need to customize the message. It better say something like "Hey Howard, we hung out at JavaOne last year." or "I attended your talk on Clojure." or something else real and personal or it's quite, quite, likely to be deleted immediately.