The premise behind their big investment round was that they would take their annotation tech to the rest of the web.

The only place I've personally seen it used is for people to snarkily mark-up political op-eds.

It's a cool vision -- I wish I could see, for a given article, the best inline comments or just those of people I find credible -- but it doesn't seem scalable to the wide web, as there just aren't enough people willing to put smart commentary on every article instead of just firing off a few tweets, commenting on the article at an aggregator like reddit (which gives a signal of which things are worth commenting on, and a built-in audience for the commentary) or writing their own post.