I'm sorry, but this is a terrible idea. If you have to bring in two new users a month, that's 24 users a year. There are probably about 24 people I know that I'

I'm sorry, but this is a terrible idea. If you have to bring in two new users a month, that's 24 users a year. There are probably about 24 people I know that I'd be ok with sending such an invite to. However, after I send out an invite to the first two during the first month, the next month those two will also have to send out two invites. Odds are that at least a few of those four invites will go to people in my potential invite pool. I'm guessing that within 4 months tops, all the people I'd want to send invites to would have already received one. Also, keep in mind that such a requirement would definitely reduce the number of users who accept invitations to join. "Oh, hey, join this free, immature site that also requires you to do their work for you (bring in new users" "Yeah, screw that, buddy." You can't force social marketing. By trying to make users spread the word, you'll end up alienating them and hastening the end for your site unless it is SO AMAZING that it dispenses kittens and blowjobs every time you sign up a new user. If you have a decent idea, you can get the word out and people will spread the news (centsports.com comes to mind, it didn't take long for most of my friends to hop on that via word of mouth).