Presently, Facebook is the place to be, simply because everybody is there.
There are many other social networks (e.g. - Google+, LinkedIn, etc.), but fewer of my contacts are members of these sites. Facebook, however, seems to have everybody: old classmates, old friends, friends of friends, former co-workers, former employers, etc.
If your purpose is to create a genuine personal community, go with Facebook.
Google+ is good too, but I find fewer people on there.
If you want to network for the sake of finding employment, join LinkedIn. It is not as flexible as Facebook, which turns off many people, but it is an excellent place to connect with potential employers and possible career contacts.