At the moment I use facebook. However, my most active account is the one that isn't my real name. I have one under my real name but didn't want to use it because it is just too easy to match message board accounts with facebook accounts if people are dropping pictures and real first names. For this reason, it's also easy for identity theft to occur. So, my real name and account there is known only to select relatives, and to strangers there is nothing to connect me to my online self. Not a single post or picture.
Under an assumed name I use FB mostly to keep up with my interests, rather than gather long lists of "friends", which, in my opinion, seems to serve as a point of vanity for some people - like people with thousands of friends. Of course I've used it to look up people as well - famous people or just people I used to know or do know. So far curiosity hasn't killed this cat.