I wrote a few days ago about the decline of Japanese biggest SNS, Mixi. I guess it is not unique to Japan, after all. This morning, San Francisco Chronicle writes about the early signs of "diaspora" from MySpace.
It is not that MySpace will go down, but looks like it will "peak out" before long. Hearing comments from "This Week In Tech" podcast, they were talking about their young daughters don't do MySpace anymore, because it is "so 20 minutes ago". Just as people migrated from Friendster to Orkut to MySpace, they may eventually migrate to Facebook or Ning or whatever new, when it is out of fashion.
If SNS is a fad like clothes and shoes, then these new sites have to make money quickly while it is fresh. Their ability to quickly gather advertisers will be critical for their survival.