The commerce pioneers proved APIs worked as business infrastructure. Flickr proved they could be the foundation of something social and creative. Flickr launched in 2004 and shipped a deliberately simple API about six months later, and that simplicity — attached to a platform people genuinely loved — set off the mashup era and gave web APIs their cultural traction.
Twitter is the one I would call the most important API in the history of this space, and I mean that in both directions. Launched in 2006, it defined what a social platform API could be, normalized third-party access as an industry model, and produced one of the richest developer ecosystems ever assembled. Then, over years, it dismantled it — restricting access, competing with its own ecosystem, changing terms, and finally pricing out nearly everyone who had built on it.
Facebook is the whole arc compressed: the platform launch, the developer explosion, data access that was far too loose, Cambridge Analytica, the lockdown, and an industry-wide reset in how everyone thought about API access to personal data.
The lesson those three teach together is that an open API ecosystem is a choice a platform makes continuously, not a state it achieves. Every one of them was genuinely open at some point. None of them stayed that way.
