A thing is worth its market-clearing price. Facebook has $15B in revenue and pays $12B in expenses (for the sake or argument, it's all payroll). I claim the workers are getting exactly what they're worth, and the remaining $3B is the value contributed by capital/management. Your claim seems to be that workers created the full $15B in value, and the $3B is being stolen from them. Apologies if you meant something else.

If that's true, Facebook's workforce could quit tomorrow, go build a competitor, and claim the full $15B for themselves. Why don't they do that?

The traditional Communist argument has been that they can't because capital owns the means of production. Factory workers would happily go start their own factory, but they can't afford the machines to fill it. That argument doesn't work too well here: the means of production needed to build Facebook are accessible even to college students. Basically an editor, a scripting language, a web server, and some linux boxes.

Another reasonable argument is that they can't because they need to eat. The time it would take between quitting their jobs at Facebook, Inc. and seeing revenue from Facebook Co-Op would exhaust their savings. That is a problem UBI can solve. It's not that UBI pays what you are worth, but that it removes the urgency that sometimes prevents people from getting their full value in the market.

This idea - that workers create all the value in a company, not just the subset they are paid for - is testable. If you're sure it's true and you want everyone else to believe it, then I think you'd want to run the experiment. UBI could be a way of doing that. Could also do grants that pay living expenses during entrepreneurial activity, or just straight-up house and feed co-op founders without asking for an equity stake.