I just saw this, interesting. Many journals/societies are discussing the enhancements they provide in addition to the papers themselves. Discussion etc. in the form of blogs about the papers, but hosted by the journal directly is compelling. No reason that libraries could not also get into this and be the place for online academic discussion, or at least a particular subset thereof.
I think these addons are great, but they are just addons. How many of them are actually shown as part of the paper itself? Or linked to FROM the paper? Unless they are part of the record of a paper, they risk disappearing…which does raise the idea that the sorts of things that are accumulated and counted by altmetrics – blog posts, news articles, etc – should be archived along with a paper if at all possible. There’s no mechanism for that now.