It's one of those ideas that never die - services to comment on sites have been around for more than a decade - I've seen a few during the dotcom boom. The problem always is lack of critical mass - the service user base is tiny compared to the vast amount of pages on the web. The chance of me finding and another user commenting on the same web page is tiny even if you have millions of users.

Consider making something sites can embed and make available to all their users instantly. For starters be THE service for a few sites instead of one of a dozen services for the whole internet. Something like a Disqus with drawing.

Also, no excuse in 2014 to use screenshots instead of the actual HTML of the site.