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.