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Based on a tried and true concept, Tweet Defense is an iPhone game that lets you fight zombies with various tower defense units, but with a social twist: it grabs your various Twitter stats - number of followers, friends and tweets - and calculates bonuses to your defense based on them. It's like an infinite time sink which connects two enormous time wasters - Twitter and tower defense. In other words, it's beautiful.
Get Tweet Defense for $0.99 from the App Store.