I can't say whether Facebook joining the fediverse (ditto Google, which won't want to miss this opportunity) will kill or absorb the fediverse, though it's obviously and rationally something to worry about.
But preemptively blocking them -- and the people already using them -- from your instance guarantees less relevance for the fediverse.
It also guarantees, if my instance joins such a boycott, that I'll find a new instance.
