At the end of last month, Facebook made a bombshell disclosure: As many as 90 million of its users may have had their so-called access tokens—which keep you logged into your account, so you don't have to sign in every time—stolen by hackers. Friday, the company put the actual number at 30 million. Here's how to see if you were one of them, and if so, what the hackers got from your account.

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