That's not entirely true, your Facebook UID always maps to your real name, gender, and your locale (country ie: en_us etc). That information in the hands of som

That's not entirely true, your Facebook UID always maps to your real name, gender, and your locale (country ie: en_us etc). That information in the hands of someone clever is more than ample to make some pretty decent guesses as to your identity, especially when you can pair it with other data that you may or may not have collected in your own app. In a previous job I held where we dealt with this sort of stuff facebook ids were generally thought of as PII.