I like analogies and for me, although the word "communities" is used to describe social networking tools, I sometimes think they are more like "Parties" i.e. events. Stay with me on this. I have been and go to a number of business parties such as business functions, corporate event after-parties, corporate hosting, etc.
I also attend personal and family social functions like kid's birthday parties, weddings, christenings, etc. At these parties, I meet various people - some of which I "connect" and/or "introduce myself" with either commercially or socially, and some of which I don't.
The analogy for me is that social networking tools - mySpace, LinkedIn, Facebook, Xing, Plaxo, Spock, etc. - are all huge online parties. I tend to 'attend' LinkedIn and Facebook parties more often as there are more people I connect to there than others. It doesn't mean I don't 'drop-in' to the other parties from time to time to see what's going on. Party on!