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Because chances are, your question is probably about something else. In addition to Facebook. Something more specific to the subset of all possible Facebook programming questions. And a tag that describes that topic goes a long way toward helping folks find it, understand it, and - hopefully - answer it...
So you gotta ask yourself, "I know my question is about Facebook, but what else is it about?"
Those are all tags that could help to better classify your question. There are many others. Use them!