How do I know it's over? A couple of things. One was when I built Texsy.com and started to promote it to bloggers. Unfortunately I found that long tail sites have no traffic. Basically, to make an ad network succeed you need BIG sites. One large site is worth, oh, two million small ones...
Second, readwriteweb has a post that uses data from Compete.com to show that contrary to Fred Wilson's intuition, the big sites are getting bigger percentages of internet attention.
It was a good story though...