Messenger messages are supposedly e2e encrypted, but that doesn't mean the clients don't then turn around and give those messages to Meta. The clients do scan the messages and are known to add that information to your advertising profile which is also sold. So, while the messages themselves might be protected in flight, and they may not be shared with Meta in full, they are not private. Also, the meta-information about who you're contacting is not encrypted, but that's also the case with most apps, including Signal, as that is difficult to pull off while still being easy for people to find you.