I run AVH First Defense Against Spam as a spam filter on my blog. When anyone tries to create a comment using either the Facebook or Twitter connections, AVH blocks it with a message like "Cheating, huh."
While it's obviously AVH that's doing the blocking, I believe it is doing it because Social is maybe creating those comments through an HTML function that makes it look like a random comment bot that's trying to send a comment to the post and circumnavigate the comment box.
Commenting goes through no problem when you just use the WP comment box, and haven't logged into any social networks. Once you've logged into a social network, it will block any comment you try and make through either the network, or through the comment form.
Finally, I don't really believe it's AVH's problem as it appears to be working correctly. Mostly wondering if this is something that can be fixed at all, or if I'm forced to pick between the two plugins. AVH blocks over 5000 spams a month, so it's not much of a contest that social will get deactivated.