It turned out there were a greater number of devs NOT at HP... and today, there are more devs NOT at Facebook.
So which company would have the resources and reasons for contributing to an entirely open social network, with no way to monetize it?
Who says this won't happen as well to an open collaboration platform? Who says companies wouldn't benefit from it, or that they need to monetize it directly? Companies contributed to Linux because they got benefits by participating in the operating system. Open source works like that; adding to a common resource is beneficial for oneself and the community at the same time.
Similarly, an open platform like matrix.org can provide huge benefits for internal communication in a company; and making public the tweaks that you create internally is a great way to influence the direction of the software, as well as making it easier to integrate the improvements made by others. This is how Linux grew to what it is today, after all.
But we've had open source alternatives for many of these things for many years, and no company has gone to support any of them.
There's no company backing mastodon, and very little support for similar tools.
Of course it could happen. But it's not happening.
If a free software collaboration platform gains traction in the enterprise, it could be pushed as a safe alternative in the next data scandal, and maybe enough people could switch to it to become a viable public resource like Linux and Apache have been since the turn of the century. It's a plausible scenario, even if it's not guaranteed to happen soon.
So far open source communities haven't really tried to clone Facebook to the level of detail required to make people feel at home, and you're already competing against 'free'. So that avenue isn't available either.
I suspect it isn't possible to compete with Facebook because for most people it isn't broken.