It seems like something that should be constrained to social network circles. It might be useful to post graffiti on any site on the Internet - within my group of friends.
Unless your friends are very, very, very similarly minded, that falls under the "everywhere you go, nobody is there" class, and you quickly stop using it in favor of some sort of centralized comment stream like a chat channel or Facebook posts.
The power-law distribution of web pages wreaks havoc on almost any attempt to distribute people well here. Your solution basically would make it so the annotation works OK on only those pages that are on the high end of the local power law distribution, but that is probably fewer pages than you might think as you'd all have different long tails.
