I think you should also consider the type of users you're targetting on FB and reddit as well. From my experience, I would've assumed that redditors would be less likely to sign up because they're already at a place (say the hiphopheads subreddit) to browse and read stuff related to hiphop, so I'd think it's less likely for them to go on to another site to read similar content. Where as on facebook it's users who are just browsing facebook and there isn't any hiphop pages for them to read on FB so if they click through, they can continue reading more about hiphop.
I would agree if IllestRhyme was about reading stuff related to hiphop. Instead, it's about _creating_ stuff related to hiphop. There's much more of a focus on user interaction. A user gets far less out of the site if all they do is read (and this is pretty obvious from the minute you get on the site).