Communities before were usually separated by platforms, because the platforms were not yet sophisticated enough to support vast communities of varying opinions. Now we have a few big players that can do this and more. Social media has turned into a war over who can blow the most filter bubbles, and with services like OpenID and "connect with facebook", everything has slowly become part of the same identity.
I'm hoping that a new platform will eventually come along to pry this apart, but it's going to take a completely different paradigm to cause it. Unless something like Google+ proves otherwise, websites like Facebook and Twitter et al have the potential to monopolize social networking.