There are many social networking sites like Facebook as we know it. But there are many other social networks that are much smaller and more specific aims have been like italki or like stackoverflow that particular computer programmers, and I love it. Which social networks are useful and interesting , you know?
None - not into that at all.
Maybe I am too old or just do not have enough people in my life to be bothered.
None - they all suck. Go outside and play instead.