Having one super community is not going to be desirable.
And Im not talking about the internet. Im talking about groups of people in general.
Different people have different interests and it is perfectly fine for them to segregate themselves by choice into like-minded groups.
I would perhaps argue that this is the entire POINT of a community. To pick and choose who you want to interact with.
The fact that these smaller communities with a narrow focus work while the larger ones fail is proof positive that tools are not the solution.