Do people really click more ads the longer they spend on Facebook? It feels like an unreasonable assumption to me. Clicking on an advert takes you away from the app, so if you design an app that people never want to stop using then they're never going to click an ad. The 'ideal' (for Facebook, not for users of Facebook) is an app that people want to use enough to be aware of an ad that's displayed on the page, but that isn't compelling enough for them to want to stay on the page rather than click on the ad to go wherever it takes them.

Facebook would maximise their profit by tailoring the application to be something that people want to log in to as regularly as possible but not to stay on for very long so they're compelled to click away after a seeing an ad. Maybe that's what Facebook have done, but when you hear about users spending hours a day scrolling through their timeline on there it doesn't seem that way.