Are we ready for vertical social networks?

I've been doing some thinking about vertical social networks (VSNs).

Generally, VSNs are the cable channels of the social networks. Rather than going wide (facebook, linkedin) across people's interests, they go narrow and deep. In retrospect, we created one such network with Upto11.net back in 2004 (though we certainly didn't think of it that way then).

You could make an argument that P2P networks are inherently a VSN (around music discovery) and a rather unique demonstration that VSNs can succeed even when everyone chooses to remain anonymous.

The questions I'm focused on at the moment are:

1. What are the drivers that cause a person to take the leap and join a VSN?
2. What's base level of value/utility have to be in place to cause someone to join one?
3. How many people already have to be there to make it worth becoming a member?
4. Do people need their "real" friends to be there already to join (especially later adopters)? or are VSNs the means for folks to make new friends who share a particular interest?
5. What's the right blend of "passer by" features vs. "member" features to encourage adoption in order to get the network effect going?

More later, off to check out dogster.