I have used them all and found very little difference in the number of comments. Unless you have a community that requires interaction to find the porn, then comments are few and far between. Even on the huge tubes where you can comment anonymously without an email, the video may have 100K views and one comment like 'she's hot'.
For what it's worth though, I like Disqus the best. No way would I use facebook comments on an adult site because no one wants their comments posted to their wall, and IntenseDebate is ugly on a site IMHO.
I actually have a site that sees about 10K uniques a day and it uses standard wordpress comments with email disabled so all they have to do is type a name and a comment and it posts. Been running like that for a couple weeks and not a single comment.
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