In terms of social sites and networking Facebook is a forefront leader followed by Twitter and Tumblr. However these other sites do not offer the same level of social interaction that Facebook offers. The only comparable site is Google+ which is scarily close but more stream lined and easier to navigate to the pages and communities of interest.
As Napiform said, "I like Google+ but none of my friends are there." Which seems to be the main reason why no one is using it. So I pose this question; Taking your friends out of the equation which site better delivers the social networking that you're looking for? Facebook or Google+?