With no comment karma or other means of emphasizing the important content, the new Facebook comments plugin is going to totally kill commenting on popular websites like TechCrunch (which, amazingly, is testing it right now. The first post got >1000 comments. With such amounts it's just a lot of noise. If you are big, do it like they do it on Slashdot or don't do it at all!
On the other hand, it can be a real killer feature for small sites that normally would have zero or very few comments, with all interactions moving to Facebooks and Twitters or the world. Now with Facebook Comments, it can stay on Facebook and you don't care as you're still getting your comments back on your blog and the more people share it and comment on it, the bigger chance you get to attract new readers. Amazing opportunity!