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.