"But a big part of our social identity is where we live, and where we live isn't captured by those other social networks."
In the first few years of Facebook it was heavily community based. One could freely view the profiles of anyone at the same university.
You could post your class schedule, and easily find someone to exchange notes with.
Facebook quickly grew beyond being a small community into a global network. I suppose it could be nice to have a community based option on the web. However, many groups have a page on FB that handles this role.