That’s a good point. However, the difference between something like this and Diaspora is that for the latter, people have to sign up again.
Here, the communities that help start it off bring their members into the network right off the bat. So if Smashboards joins, then suddenly you’ve got 252,000+ users. If The Admin Zone joins, that’s an extra 21,000 members. Etc.
Of course, that switches the issue from ‘how do I get lots of people to join’ to ‘how do I get lots of communities to join’. But I suspect the latter is a slightly easier problem to overcome than the former.