The Register can be hit or miss, but this is a great article. I like the points on "Preaching to the converted", "Bogus numbers" and "Community land grab."
The last point is probably the most relevant to what we see happening today. How many social networking sites are going to be built up, then bought, then torn down before we're out of the social networking frenzy? We've had 3 or four major ones since Friendster launched the genre. Plus, now every startup is trying to latch their product onto social networking.
I'm also curious... how much money has been made across all of these social networking sites in the last year? MySpace's revenue was $200m last year, and they're probably the bulk of the revenue. It seems like an awful lot of hype and VC is being aimed at a very small market -- that's a good indication of a bubble.
Anyway, check out the linked article.