Maybe it's just me, but I'm completely ad-blind when I'm on an SNS like Facebook. It's not where I go to be advertised to in the usual way. Now, if a company makes an intriguing business business page, and gives me information I'm interested in, then I'll look at that. What I won't look at are ads pushed into my stream or sidebars. Essentially, they're doing it wrong.
I like Marshall's idea of paid business pages instead. That makes sense on a number of levels.