Edit: this comment changed my mind. In a nutshell, if we can't keep a large instance controlled by "the enemy" from destroying what we've got, then we just have to do better next time.

My instinct and ideology tells me that no, we should not make that decision before it whatever Meta does becomes reality.

Fortunately, I have laboured long and hard to develop a scientific frame of mind, where decisions are based not on instinct and ideology, but on the evidence of reality, even in the face of incomplete information.

TLDR: we should not federate with Meta and that should be the decision taken by the majority of instances. Not because it's Meta, but because of the power that comes with a having a "controlling interest."

This is a network of independent actors. Other such networks are publicly traded companies, condo developments, and most cryptocurrencies. Whenever you have such a structure, you must be vigilant against the 51% problem, because whoever controls the majority, controls all.

This is why publicly traded companies do stock buybacks when they have extra cash and the company holds less than 50% of outstanding stock.

This is why condo developers (especially in condo conversions) like to hold the majority of units as rental units. That way they dominate the vote, and do so in their favour.

This is why cryptocurrencies must be vigilant against "51% attacks."

Also, the reality is that it is frequently unnecessary to actually hold 51% to gain control. In a system of widely dispersed small holdings, it may be possible to have de facto control with much smaller percentages. If the typical holding is less than 1%, one actor holding 30% might gain effective control with a minimum of effort. Think of them being a "super swing voter" whose interests must be satisfied in order to get things done.

If you've read this far, then you can probably guess that my real point is that we must be vigilant against any instance becoming too large. Not because they are untrustworthy or bad citizens, but because we have little recourse should they decide to exercise the power they have. In other words, Meta is just a specific case of the general problem.