[beagle3]

Your DOB was likely provided by some external source. I never put my DOB in (they didn't require it in 2004, when I created my account - which I do not actually use), and it materialized there at some point; other data I did not put in also materialized, and most of it is wrong (about schools I went to, etc).

I suspect they picked it up from someone's contacts uploaded by the facebook/whatsapp/instagram app, but have no way to know.

They are also buying data from data merchants (which in the US includes your phone records, credit card purchases, and more) and integrating it.

Technically, the war has been lost a decade or so ago; Facebook co-opted everyone in your life - your friends, your bank, your phone company, etc - to spy on you. If you didn't tell THEM the wrong DOB, you are out of luck with facebook.

Legally, it may still be outlawed ... but the horses have left the barn. Facebook, every 3 letter agency, and quite a few criminal enterprises have a copy of everything Facebook ever knew about you.