Think of companies like beverage giants, car manufacturers, large consumer products giants, financial institutions etc or almost anyone who advertizes on superbowl on TV. Their primary objective is to build awareness & a certain brand perception among a large segment of consumers so that when you buy their category, you will choose their brand. Coca Cola doesn't necessarily want you to buy it that instant online you see the ad (though if you do, it's a nice bonus). Rather the objective is to build an equity that will make you think of yourself as a "Coke person" instead of say "Pepsi person".
Remember that bulk of the advertizing money in the world comes from such players. They pay facebook a premium for being able to deterministically show your advertizing to a given target audience.
Other small business ads such as local coffee shops, restaurants, etc. that advertise deals/coupons also convert very well and get shared like crazy.
Note that, people generally don't buy things the first time they see a banner about it. The process of today's digital marketing involves nurturing at multiple levels. For eg, I learned about the brand from Facebook. Then, I stumbled upon a blog post written by them. "Wow. Who are these guys? Why do I keep hearing about them?". I learned more about them and maybe bought something from them after a while. Undoubtedly, Facebook uninterrupted ads will (have?) become a major part of this funnel.
What does continue to amaze me is that users tolerate how much data FB collects, how they use the data, how duplicitous they have been over time with users, and how they manipulate your feeds/user experience to benefit their revenue/advertising income. But I have come to realize most people just don't seem to care about privacy that much.
On sites like Facebook? Yes. There's a lot of criticism, much of it justified, about spammy ads online. However, with a system like Facebook's or something like a search engine, the ads being shown are normally for something that might genuinely interest the viewer. That makes a favourable reaction much more likely, and it means far fewer resources are wasted showing ads to people who aren't likely to be interested.
I still haven't managed to wrap my head around how exactly facebook makes money. Do so many people actually click on ads? And this somehow makes billions? I find it very hard to imagine (for myself).