The advertisement industry moves an insane amount of money. People pay up to several dollars for a single potential customer ending up on their website(s).
Ads for lawyers have ridiculously high prices in Adwords. Since my first job was in a firm that handled marketing for personal injury lawyers, I had an impression that adwords was really expensive and ws wondering why the ads were so ubiquitous for low revenue businesses too.
FB knows so much about you. You age, your likes, your friends, your location, etc. This allows for very precise targeting of ads compared with say a billboard. This combined with the amount of time it's billion plus users spend on the site.
Advertisement platforms extend beyond single platforms. Google Ads are on every website in the Internet, and Facebook ads are on other sites too. So even if you don't have a Facebook account and never visit Facebook itself, you will see their ads and they will start following you around and start collecting data on you.
(Check out the pricing on Adwords for serious medical conditions some time.)