when you let somebody login to your site with their FB login....
 

what's the benefit to you as a siteowner? do you get to post to their wall? get access to their friends?

is there a plugin for VBull that allows people to post to a forum with their FB login rather than registering on your forum?

yep

more potential for user interaction if they feel like don't have to go through a registration process

benefit as a site owner is that you're more likely to pull in their true primary user info (name, email etc) versus some alt account they set up

i dont use it for adult though only mainstream

cant answer the VBulletin stuff

As a customer, it freaks me out so I avoid those sites. I am surprised it adds to signups.

Convenience. Also, you can have comments they post on your site, post back to their profile and it will show as being from your site with a link to your site.