I think this is already happening. There are successful niche social networks broken up along at least two lines: 1) Geography. VZ-Networks' sites are bigger t

I think this is already happening. There are successful niche social networks broken up along at least two lines: 1) Geography. VZ-Networks' sites are bigger than Facebook in Germany, Vkontakte seems to still be #1 in Russia. 2) Activity. CouchSurfing.org and meetup.com have completely different use cases than Facebook and are thriving.