One of the challenges online communities have is controlling their growth while maintaining quality. Quora had to revert to ‘invite only’ after the quality of the site began going downhill
I really like how forrst has decided to deal with this situation. First they limit the number of invites you have then they make the responsibility of bringing in new members that of the old members and make inviters publicly accountable for invitees.
Now in my dealings on forrst, I am aware that Joel’s reputation is at stake and I will not mess that up.
Of course this method reduces the viral coefficient of a website because I will not be importing my address book.
That is a good thing.