Facebook, has among many choices, two clear and easily territories in which to advance user privacy: technically and legally.
Technically, Facebook could implement E2E for private messages. Doing so would make disclosure not something they were unwilling to do, but unable to do.
The other, and this is much more juicy, would be to lobby the California legislature to make it illegal under state law to disclose private conversations re: abortion to states where abortion is prohibited.
The fact that these are possible, but not likely to happen is another issue altogether.