AOL's core proposition was being better than the Internet (more curated, coherent, and faster). When the web and internet exploded in size and scale, AOL's value evaporated. The dumb mergers and other mistakes were window dressing on this landscape transformation.
And for years (decades? still?) afterwards, people used AOL Instant Messager (AIM), because it was the most network/platform component of AOL.
So how would that happen to Facebook, and what would it look like?
Users would need an order of magnitude superior alternative, and most critically, users would need to move en mass. Facebook has rightly identified onboarding younger cohorts as key to their survival, but I don't see any realistic way Facebook dies a natural death in under 40 years.