Very true, jscore, referring people to Skritter has no network effect at all. We do offer extra subscription time to both the recommending party and the new ref

Very true, jscore, referring people to Skritter has no network effect at all. We do offer extra subscription time to both the recommending party and the new referral, and we spent a long time trying to figure out the right mix of benefits for both, but I think you're right that without the networking effect there's a lot less interest to recommend people. As you say, it doesn't help you to recommend anyone to a site where the primary activity is independent learning. Now, we could alter that and some social features, but my best guess is that it's not just "social features," because tacking that stuff onto Skritter probably wouldn't help too much! To get maximum gain, we would have needed Skritter to be more easily spreadable from the get-go. :)