In 2011, Facebook was good at its job, with a chronological news feed, meaningful notifications, and a messenger paradigm that worked much like AIM in the early 2000s -- you could see who was online, away, go invisible, etc. In 2018, Facebook has lost all of those positives, and with it a lot of user engagement, all for the sake of cramming the maximum number of ads down the users throats. And I'm not even getting into the nasty dark UX patterns, like hiding the ability to delete your profile (seriously, there isn't a link to do this anywhere on the site -- you need to search Google for it and they change the link all the time to break external guides) and showing random pictures of "friends who will miss you if you leave!" when you try to delete your account. I don't think your average user would mind leaving Facebook much if there was any actual alternative.
This is just a minor quibble to lambda_lover's point, which still stands. Facebook may understand it's job is to shove as many ads in its users' faces as they will tolerate, but that's not why users use its app. It's users view its job as being useful to them, and being bad at that makes makes FB vulnerable to competitors or even just general dissatisfaction.