When talking about social-networking sites and whether the larger sites can keep their market share from eroding, the answer to me, is no they can't.
While a place my MySpace and other big sites will still grab millions, it will still slowly degenerate in users like network television has since the days cable became a competitor.
General types of sites will always attract people for general use, but when anything specific is desired, they will always go somewhere that 
attracts like-minded people. That's the world we've been living in for years, and it's not going to change.
One writer recently said that people are starting to join multiple networking sites, and have been for a while. At this time it doesn't seem that it has totally made a person quit one for the other. Eventually it will have to though.
The reason why is that we only have so much time to do what we want. When one thing is chosen, another thing must be let go; even if it's the amount of time.
MySpace, and other large sites, are the equivalent of television right as cable entered in. There will be a continual fragmentation as people gravitate toward networks that entail something that they're passionate about. Consider that as you're thinking of something to build online ... niche networks will be exploding in numbers for years to come. There's a piece of the action waiting for those who tap into it.
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