An obvious way to make money: Persistent reputation is extremely valuable. Extended and deep information about a user from their writings and the sites they visit is extremely valuable. Think about the targeted advertising down the long tail, where eCPM will be huge.
I'd hope their plan is more than to reach some critical mass then start inserting ads into people's blogs. Will it be "put our ads on your blog, and we'll split the profits" or "pay for Disqus or we'll put ads on your site"? I guess we'll wait and see, perhaps they'll manage to productise it, if they've not reached the maximum price point yet.
