We get a lot of spam comments on Bald Move. Probably 30% of the emailers and about 90% of the members who sign up for the site are spam accounts. I've been looking for a good way to solve that problem for a long time. Facebook comments seem to be a good solution.
Things we tried to limit spam comments
The first thing I tried to solve the spam problem was to moderate the first comment from each new user. Once they had their first comment approved, they were free to post comments in the future.
This solution definitely works but not only does it require me to mark each spam comment as such, but it's a terrible experience for the user. When a user goes to post a comment on a website, they expect it to show up immediately. When it doesn't, they get confused and / or discouraged from posting their thoughts in the future. That's unacceptable if you're trying to build a community online.
Then I tried adding a captcha to the site's sign up form. That's also not an ideal solution since captchas make it much more annoying for users to sign up for an account. It's much less annoying than having your comment sent to a moderation queue for a few days, however. The trouble is, there isn't a great captcha plugin out there for WordPress. I'm sure I could implement Google's recaptcha which seems to work pretty well but there is a much better solution.
Benefits of using Facebook commenting
Facebook comments allow us to use Facebook's own spam controls as our own. They have teams of engineers dedicated to keeping spam accounts from signing up and posting comments on posts. Facebook does a far better job of this than any WordPress plugin ever will.
By far though, the biggest benefit of using Facebook for our commenting system is that it allows anyone with a Facebook account to comment immediately, with no barrier. That's huge. Nearly everyone already has a Facebook account. So instead of limiting our pool of commenters to those willing to fill out a captcha and return after having their comments moderated, it opens it up to entire user base of Facebook.