I agree with Ryan for the most part. Here at SEOmoz we get so many comments for each post that if we moderated each comment, it would seriously kill the conversation. However we have a team of people who watch for spam, plus we do have moderation set up for people who have less than 25 Mozpoints and have a link in their comment. At that point one of the team members has to approve the comment, remove a spammy link or delete the comment all together.
In general I'm not a fan of moderating especially if the person moderating checks comments infrequently. Nothing bothers me more than leaving a well thought out comment on someone's blog post in reply to the post itself or another comment and it taking 2 days to finally show up on the site. I wouldn't bother commenting on that blog again.
There's a fine line between moderating and keeping the spam out and if you can find that you're golden.
As for Facebook comments, I haven't personally implemented them on any sites but many people who use them have really loved it. Ryan linked to my post on using them and it seems like a great marketing/virality tool for sure. Facebook gives you easy moderation as well and I think it cuts back on spam quite a bit because it has to be a real user logged into Facebook in order to comment.
Hope this helps!