There's the competitive edge, too. Lots of people (myself included) are very competitive and want to "win." More twitter followers, more foursquare badges, etc. If you and a friend both join the same site that promotes these types of game mechanics, you (potentially) just got two loyal users who will fight to beat each other, also inviting their friends to compete as well.
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Some great points here, I've always noticed that sites with "game mechanics" have a tendency to make me addicted, even if the content is mediocre.
There's the competitive edge, too. Lots of people (myself included) are very competitive and want to "win." More twitter followers, more foursquare badges, etc. If you and a friend both join the same site that promotes these types of game mechanics, you (potentially) just got two loyal users who will fight to beat each other, also inviting their friends to compete as well.