How Many Social Network Sites Do We Need?

I keep discovering "social networking" sites that hardly have any members, even those that have been around for several years. And I don't mean industry-specific or niche sites, but wide open networks. Are they not marketing them, or are they just truly lame?

Another thing I've learned is that most social networking sites have a majority of fly-by visitors, and generally less than 40% regular users. So it would appear that many people are curious about the current social networking hype and go check out these sites, yet few of them find reasons to come back. Look at it this way, if a site claims a million visitors a month, you can bet that at least 60% of those are one-time visitors who will never return. Facebook and MySpace are solid, with 55-60% regular users, and 25-35% fly-by visitors. The remaining percentages are hard-core users who are on every single day.

I'm assuming in the next year or two there will be a purging of these sites, where a few of the successful ones will buy the decent smaller ones, and the others will simply disappear because they never worked. Just like tech companies in 2000, and banks in 2009.