I've always thought some of those sites - the ones that offer free big screen TVs and the like - could be an effective tool for small groups. Most of them require a total of six people or so to complete, and the initial person gets the reward. So, if happen to live with five friends, it's easy enough to have one person sign up, and the five friends be the "referrals". Use parents addresses or whatever, and your house has a new TV!
Outside of that relatively limited sample, I really don't see them as viable. Possible, sure, but not terribly realistically.