New FB users are shocked when they register to get all these (sometimes) accurate suggestions for "friends". FB does this by storing all historical name searches done by any user. If the search target finally does register, FB "knows" all the people who have searched before for that person. It's just programming, no need to get spooked ! If you don't like it, then stay away from FB - it's not compulsory, but can be very useful in Family History if you understand and set your privacy rules.
So true.
It is not just Facebook.
Google, knows more about you than you do, your identity is known to them, all your searches are known, all the sites you visit are known, your movements are known.
Major Supermarkets know more about you and your buying preferences than you do.
Your mobile phone company knows who you call, hence your social grouping, they know where your phone is, so again more data on you is held and processed.
Your broardband company knows your online history.
You simply cannot escape data being collected about you.
Data is power and many companies like Cambridge Analytica collect, process, manipulate and sell your data.
Burying head in sand does not stop data being collected.