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Facebook explicitly collects your birth date though, doesn't it? (Question is both rhetorical and actual.)

• danShumway 8y ago
  And gives you explicit warnings not to lie about it. And limits the total number of times you can change it (so you'd better hope you lied when you signed up 5 or 10 years ago). Facebook is not real big on the 'ol anonymity. In practice they can't really stop you from lying, but just telling people not to lie and making the question sound serious can often be enough to stop the average person. I suspect most people put in their correct age. Odds are also pretty good your friends are going to mention your age at some point while wishing you Happy Birthday. If Facebook's data collection is smart enough it could decide to trust them over you. I kind of doubt that it is smart enough to do that right now, but it probably wouldn't be the hardest thing in the world to do if people started commonly lying.