For Facebook, exclusivity was a feature because it was directly related to craved social status (belonging to an elite university). For GMail, exclusivity worked because it was directly related to craved features (large capacity for email attachments).

I don't see "lack of ads" as particularly craved by users. My wife actively wants to see ads (to the suffering of my credit card). Unless they find something else to latch on to, this will end up being yet another also-ran social.