A community is built around a premise. The premise behind Match.com is clear: meeting people for dating and marriage. Similarly, the premise behind Friendster and Facebook are easily discernible; Facebook is wildly popular in its target community, college students.
These so-called social-networking sites start with the premise, and try to build a community around it. There are a lot of other community sites that you've never heard of where the premise might be reasonable, but the site doesn't catch fire, for whatever reason. I think of these as speculative communities, where the premise is established (fans of "The West Wing", perhaps) and an attempt is made to establish a community around the premise.
The difference between a real community and a premise for a community is subtle, but critical.
Consider the "community" of people who are interested in Harley-Davidson motorcycles. There is, conceptually, a single, large community around this single premise. There's even a word for it: "bikers". But is it really a community? It's actually a large number of individual real communities. Local chapters of the Harley Owners' Group club. A dozen or so folks who congregate at Alice's Restaurant on Saturday mornings. The set of members of an online community, perhaps....
So there may be a single "virtual community" (the premise), but there are likely many splintered real communities, some of which overlap, some of which do not.
A real community that already exists, that needs a better way to communicate, is a much more tangible thing, and target for software, than just a premise.