When those Facebook users do in fact click and make purchases it is often (not always) a profitable exchange for the companies advertising. Those companies take the revenue from the sales and use it to pay people to do other work for them. Wherever it is you work, they probably advertise on Facebook.
Creating a profitable revenue engine for a million companies is absurdly useful.
Yeah, I would like to market pre-Facebook. In exactly the same way I wouldn't want to advertise with Stalin, no matter how effective his apparatus. I bet you there were people who couldn't understand that either.
My life is a painting. I already have brushes and paint, and while I could of course use more and better brushes and paint, there are also those offers to shit all over my painting for a truckload of generic, lame colours, and thousands of stamps with all sorts of shapes designed by "professionals", because making those with your own brush is just too hard. I say no to that, that is a net negative value for me. I don't live to earn money, I earn money to live. FB is right out. As the song goes: you might win some but you just lost one. Nothing more, but also never anything less.