That wouldn't be the foundation of Facebook, that would be Facebook introducing the algorithmic timeline. Remember that Facebook explicitly considered it a success because it increased "engagement" while the vast majority of its users reacted negatively to it and when commenting on it indicated that it made them feel worse, that it negatively transformed the kind of social interactions they had on the platform and that it was detrimental to their mental health (because previously Facebook had been centered on 1-to-1 and many-to-1 interactions between peers and now was about 1-to-many interactions with an audience - something I guess Google tried to mitigate in its own social media experiment somewhat unsuccessfully by letting you group your "friends" into "circles").

The revolutionary change that made Facebook uniquely successful wasn't being a social media platform, it was forcing its users (who were so far treating it as a way to keep in touch with acquaintances, old friends and distant family) to compete for each other's attention and offering corporations the opportunity to join that competition - all the while retaining the messaging that the platform is about "social" interactions between peers. And of course mining the everliving #### out of their users' data while non-consensually tracking them across the entire web without their knowledge.

But the "attention is the currency in the marketplace of ideas" concept they launched pretty much defined all "social media" companies from that point on, which is why we nowadays often forget the term used to be much more appropriate in the past (although often constrained to a crowd of very technical nerds).

Oh, and of course they very successfully killed much of the tradition of the Open Web by encouraging a walled garden approach even when it required them to actively defraud their advertisers by lying about the performance of video content. But I think the trophy for launching that extinction event belongs to Apple when they pivoted away from the original web-first concept for the iPhone to the proprietary App Store.