This is pretty clearly not the case. Apple and Facebook are companies, not platforms. The platforms these companies develop are wildly successful and loved by developers.

I think you may be too focused on how developers decide a platform that is already successful. In that case, yes, which one is already adopted the most deserves strong consideration. However, when it comes to actually achieving adoption, it’s the platform that developers flock to that tend to succeed, all else being equal. We are talking about relatively fresh terrain here. There isn’t already a huge federated system like this that has a lot of mainstream adoption.