The industry is filled with hugely interesting technical challenges even if you're not at FAANG scale. Trivial things like building a cost efficient delivery infrastructure to reducing the number of steps between receiving that first page load to ad delivery and then on to analytics. A lot of it is just math and it's fun when you structure your challenges that way. It's like inverse Pinky and the Brain everyday. We're not scheming to erode everyone's privacy, we're just lab rats looking for the shortest route from one end of the maze to the other.
I also left after a few years because all that maze running got boring eventually and I decided I'd like something with a little bit more meaning. Or, from another point of view, those years eroding everyone's privacy has enabled me to climb high enough up on Maslow's pyramid such that I now have the comfort to worry about self actualisation.
edit:
There were a few people that totally drank the kool aid, and believed that what we were doing was enabling people to gain a livelihood by running a popular website. Usually the really young ones or those at management level.
Those from a developed country were mostly interested in the technical problems and improving their engineering skills.
Those from developing countries were most definitely in it for the money.