Prima facie it sucks but they can probably get global coverage for their product/services they're already selling (trademark registration requires current use, someone said the OP company have a trademark registered) on the basis of "what do we do now Facebook are using our name".
It seems likely that if they can't leverage this for financial benefit, in a forthright way, then they were going to fail anyway.
(and frankly it's still a bad name because it's a super common word. it was doomed as a product name anyway.)
It would even appear (from the letter) that facebook was already aware of this name conflict and decided to go ahead with the rename anyways.
Also Meta is a terrible name - for both companies., but that's beside the point (but it's like really stupid).
That doesn't matter, they own the trademark.
I said the same thing about matrix.org back in ~2015 when I first heard of the project. Fast forward to now, though, and "Matrix Room" links are ubiquitous among open source projects....