I think they were mostly using their market dominance to kill apps that decided to live on the Facebook platform in the first place.

If you build an app that relies solely on Facebook login and access to Facebook data as key parts of your business model then are you really a competitor?

Well, if you build an app that runs on Windows like Netscape did, are you really a competitor of Microsoft? Turns out, you might be, if Microsoft want to make their own app. Similarly, Facebook might want to introduce or change native functionality, or monetize their platform, in a way that competes with your app.

That's a good point of comparison but the difference I think is at the time Windows was a dominant OS, and most people's only option. For them to come out and compete with Netscape and prevent Netscape from operating on their OS would be a death sentence for Netscape because it is not reasonable for Netscape's response to be "Fine we'll make our own OS!".

There's a huge difference between "build your own OS" and "build your own website login".

It's not a very tall order to create a social website that doesn't use FB login. Social websites should not by default have a right to all of facebook's users. They should have to build that user base themselves.