It's not only about changing history, it's about getting transactions mined in the first place: censorship-resistance. In Bitcoin, as long as you pay the fee,

It's not only about changing history, it's about getting transactions mined in the first place: censorship-resistance. In Bitcoin, as long as you pay the fee, some miner will probably mine your transaction, based on purely economic incentive. Miners that refuse to serve certain customers based on "tainted" coins or illegal activity simply forego that revenue to miners who don't. I'm going to go out on a limb here and conjecture that Facebook at the 100 entrenched industry players who are part of this new system will probably not consider censorship-resistance as a high-priority goal for the system. They will probably have a law-enforcement login portal that calls up Facebook's data profile of participants in a suspect transaction. There will probably be blacklists and fund freezes. There will probably be State Department liaisons to help comply with sanctions.