I've seen email notifications, but to me they are just as bad as subscribing to a lot of sites by email. That's kind of what RSS is for: aggregation. So yes, it means you don't have to click through sites, I accept that, but still not a good user experience. If you mean something else, let me know, I am interested.

I don't know what you mean by splitting the user base or constructing a parallel comment system. I'm just talking about how to provide commenting, as a content publisher. RSS + on-site page by page commenting (even with email notifications) has a fraction of the engagement (in my anecdotal experience) of FB.