Facebook and Twitter genuinely had plans to be platforms, remember? Paul Graham even called it a new core protocol, and YC recruited startups specifically to build onto it.
...It didn't work out that way. The "platform" analogy doesn't really hold. What did emerge was one of two things. One model is the "portal" concept, of dotcom buzz. Google & Facebook are portals to content, and they have all the power/profit that comes with controlling access... AKA, exactly what we were afraid ISPs and AOL-like efforts would try to do... the reason why "net neutrality" got a name. The other model is private marketplaces: Youtube, amazon, spotify, etc. You make the rules, cream the market and tax the participants.
Stripe is a portal, sort of. They're a lot like Visa.
I think 80s/90s operating system wars an anomaly that led smart people at that time to think private companies could be real platforms. PG even wrestles with the problem in his short not on the topic re: twitter. Modern, locked down app store OSs are the normal outcome. The "ecosystems" exist to support the portal, not the other way around.