Facebook Messenger offers optional end to end encryption just like Matrix. Just like Matrix, the server knows who you're talking to, what groups your in, who else is in those groups, how many messages you sent to which group, who's messages you react to, etc. But the actual text of the message is technically encrypted so Facebook can't respond to subpoenas for your messages. I use Facebook Messenger as an example because Facebook is (correctly) generally considered not private or safe.