Definitely Facebook. Go to http://www.alexa.com/siteinfo/facebook.com and compare it's traffic (Traffic Stats tab) with myspace.com, twitter.com and hi5.com.
Facebook certainly the biggest social networking with most members, the first populer is myspace then Friendster and now facebook, Twitter even now there can not be beat facebook
Why you think twitter, twitter services are microblogging..i think facebook is better than twitter and anyother.
I would say Facebook and Twitter in combination. If you can manage two, use these... if you can only manage 1, use Twitter. As a third option, you'll want to use Youtube.
it is one and only facebook. its really amazing social networking site.and people love in it. in present time facebook is the best.