I don't think I was clear enough on how I imagine this working. I wouldn't have it work where you can look at the browser history of any of your friends, or as a feed.

Instead, I would use any of my social-news aggregators normally, but if I end up on a page that a friend has also ended up on, It would be fun to be able to see that, and maybe I could leave tiny comments scattered around the web, visible only to my peers who come across it, and we can annotate together.

That makes me think about Genius. I remember seeing them work on this a bit, but it was a couple years ago IIRC.