Do you prefer a big social network or 1,000 small ones?

In a previous blog post on social network fragmentation, I wrote that huge social networks like Myspace and Facebook will eventually break into lots of smaller networks with specialized interests.  In the comments, Mike wrote this weekend that he disagrees.  He sees a lot more activity in small Facebook groups than in other groups built specifically for narrow interests:

I have been involved in both, private organizational networks (wiki, ning, etc), as well as groups on Facebook. And hands down, Groups on facebook generate much more interaction, compared to the private groups I have been a part of. I have one "Private Group" with over 250 members (on Ning), and there is ZERO interaction amongst the group. It is a rarity for the members to post (or even stop by). However, I have a few smaller Facebook groups, with tons of interaction daily. Why is this? In my opinion, Facebook has become part of the daily routine for many of us. People are logging into their facebook accounts daily, to see what is going on with their friends. It is a habit we are all getting used to (check email, check facebook, check bank account, etc). Nobody is logging into their private sites, because it is not part of their daily routine. It is an afterthought. The private site traffic I get is about 10 visitors a week (out of 250 members), with very rare postings. Facebook is the exact opposite. Complete Interaction. Complete Conversation.

It's an interesting debate -- so I wonder what you think?  Is there a place for narrowly focused social networks? Or is the future of fragmentation bound to take place in groups on huge networks like Facebook?  What are the pros and cons of each?