There is a psychological factor to this that is used by games as well. People get addicted to collecting things whether they be tweets, comments, etc. I highly recommend Amy Jo Kim's video where she shows examples of this across several social apps:<p><a href="http://www.youtube.com/watch?v=ihUt-163gZI" rel="nofollow">http://www.youtube.com/watch?v=ihUt-163gZI</a>