Something several of my friends have recommended for me to try, is to artificially limit my already free product under the premise that people want more of what

Something several of my friends have recommended for me to try, is to artificially limit my already free product under the premise that people want more of what they can't have. In this article He goes from 15 to 20 bingo cards for free, which makes sense since the alternative is buying bingo cards. But has anyone seen anything like that work effectively for a product that only has one level (i.e. everything is free)? Some suggestions i've gotten is to say users can generate 15 widgets per month, but can get unlimited if...they follow us on twitter, facebook friend us, email us to a friend...etc. At the end of the day has anyone seen a case where limiting a free and unlimited product actually increase the number of users?