One way is through re-targetting. You visit an e-commerce site looking for a pair of shoes. You browse for a while but nothing seems interesting so you leave. The site then targets you through Facebook with tailor made ads and probably limited time discounts. They can track you through Facebook pixel. It works like a charm because the need to buy the product never went away.

A large portion of users do indeed block ads, but most don't. The higher blocking percentage I've seen is in the region of 30%-35% (it's in Greece I think).