Maggie Fox has some familiar thoughts on How Social Media is Changing Everything:
Now, because of the Internet, and accelerated by things like blogs and social networking sites, we’re connecting with people not based on where we live, but based on what we’re interested in. Hence, the shift of importance to communities of interest.
I like this take on how and why communities of interest have grown with the expansion of social media. I particularly like the thoughts about how one (or a company) could jump into the world of social media and communities: Listen, Participate, Influence, and Act. Of course, it isn't strictly linear - there may be a lot of listening and participating before you get to influencing the community to act. But there's little chance you are going to influence without first being a participant / listener.