To put it frankly, the edit came from your account on an existing login session from a couple days before. Past that, there`s really not anything else we`d be able to tell you about what happened here on Stack Overflow that you don`t already know happened. Keep in mind that since you are accessing the site using Facebook credentials, you need to make sure your Facebook account is secure as well. If you left yourself logged into Facebook somewhere or your Facebook account got compromised, another user could easily access your Stack Overflow account by using your Facebook session. All they`d have to do is log in and select Facebook and your already-logged-in Facebook account can quickly get them into your Stack Overflow account.
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