I believe Facebook is very perfected form of Dopamine delivery and it's leveraging the attention deficit behavior of entire generations. Could it fizzle out the way you anticipate? We have seen it before but if you look at Zuckerberg's ambitious vision for Facebook and the new verticals he plans to enter/trail-blaze then there's reason to assume the goal is not to ride on Facebook's success but to continue aiming at the next place the population will look for it's fix.
If you look at some of early data and research about the effect fake news spread via Facebook has had you can see Facebook has itself firmly planted in millions of American's everyday life to the extent where it's platform can distort the very perception it's citizens have of themselves. Certainly not all but healthy chunk, large enough chunk for there to be real effects.
As far as market saturation, again I'd point to Zuckerberg's heavy investment in VR and plans to invest in more emerging verticals going forward. Not to mention that Google has been at market saturation, especially in America, for many years and we do not see it's ad revenue waning in any substantial way. And Google took that ad revenue and put it into various risky forms of R&D that has now clearly lead to potentially enormous new growth verticals like Waymo.
I'm not an avid Facebook user but I get roped back in by push notifications of someone mentioning me or tagging me in a picture. In addition, about a year ago I got so tired of only seeing "mostly older people and Walmart shoppers, posting angry bumper sticker nonsense about Obama or Trump" that I searched for my favorite tech, science and political media sources on Facebook and followed them so now my feed is of more utility to me but still not ideal.