Oh it works both ways. It's the opposite of the Apache 2 grant for example (which is extremely liberal, and I like it) but that's also exactly what you'd expect from many large companies.
There's a reason that BSD/MIT is preferred by companies vs Apache 2 (which has been adopted, yes, but much less). It's because - just like here - it preserves all their patent rights.
So yes, you now have less protection but FB has just the same if not more.
Don't get me wrong: the patents grant wasn't a great solution but it solved specific problems and concerns on both sides that BSD by itself doesn't even try to address and alternatives (like Apache 2) only address half of.
There's a reason that BSD/MIT is preferred by companies vs Apache 2 (which has been adopted, yes, but much less). It's because - just like here - it preserves all their patent rights.
So yes, you now have less protection but FB has just the same if not more.
Don't get me wrong: the patents grant wasn't a great solution but it solved specific problems and concerns on both sides that BSD by itself doesn't even try to address and alternatives (like Apache 2) only address half of.