Anything I as a solo or mostly-solo dev would put together to sell would almost certainly be sold to a company less than 0.1% Microsoft's size - even at the 1% level it's just as likely that a company would decide they'd rather just build their own version or get someone to do so as a work for hire.

Edit: I'm also mentally picturing scenarios where Facebook's terms might end up tested in court. A developer team using an open-source framework without understanding the legal ramifications is one thing, and a thing done by people not authorized to negotiate binding contracts for the company. A purchase of a company or significant software (not off-the-shelf) on the other hand is by definition done by someone with authority to negotiate binding contracts. It's not that hard to imagine a scenario where that difference becomes relevant in court. Heck, I wouldn't be surprised if it turned out that Facebook had an intern or two somewhere focused on identifying and tracking projects using React and what companies they're owned by, just in case they end up in a conflict with someone and need to say "Hey, are they using anything of ours that can make this go away?"