I'd probably add that there is a place for a (limited) number if different sites that end up, whether by design or natural evolution, tending to cater to different audiences for different purposes. LinkedIn is probably the best example of this. Irkutsk in its own way is arguably another. That said, social networks have, we'll, strong network effects so they tend towards winner-takes-all.
Definitely worth reading by http://news.cnet.com/8301-1023_3-57556344-93/the-crazy-truth-google-can-thrive-alongside-facebook/: Why can be a success even if it doesn't actually extinguish Facebook.
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