A few others have commented below, but from a top level perspective I'd argue that taking a relatively small platform like the fediverse and smushing it with Facebooks user base would dilute the community / environment.
If meta federates with everyone, it'll be a huge uplift in quantity as new people start posting, but the quality I feel would be impacted.
I imagine it would be like pouring a small cup of liquid and a big container of liquid into the same bowl, one would definitely dilute the other.