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FarmVille has been a breakout hit, with its active user count growing by around 10 million users per month. Note that this isn't simply signups: This is a count of people using the app at least once in the 30-day span.
FarmVille's daily active user count is currently 29.7 million -- that's almost 30 million people using FarmVille every day.
FarmVille's parent company, Zynga, was valued at 1 billion dollars late last year.