Ask HN: Have any of you heard about Blastoff?

There's A LOT of buzz I'm hearing from multiple social circles and casual cafe chat about this service. I've done some research on it, and am not quite sure what they're going for. They seem a bit scammy, as it's based on cash back and referrals, but the grassroots marketing efforts these guys have done seems pretty remarkable. I have tens of friends a day signing up and waiting for invites - friends who otherwise wouldn't care about online services, and receiving invites, other than Facebook - and yet I have not seen one commercial/advert/discussion both online and on tv. Coincidentally, I just received two Facebook notifications of friends spreading their referral links.