It's interesting that the Huffington post went the other way, just this week. Dropping their in-house commenting system in favour of facebook-comments:
http://www.huffingtonpost.com/otto-toth/were-moving-the-conv...
http://www.huffingtonpost.com/otto-toth/were-moving-the-conv...
There are some self-hosted systems out there, including my trivial e-comments code: https://github.com/skx/e-comments/
I remember a few years ago whenever you clicked on a Guardian news link in Facebook it would try and get you to sign up to their Facebook app/plugin, that would then publish on your timeline what you'd been reading, let alone commenting on. I thought that was a bit ghastly.
I'm assuming that when you make a comment through Facebook on the Huff post, it displays on your timeline or some such, providing additional reach for the story.