Well, let's be fair. The threat model for end-to-end encryption assumes that endpoint devices or the software itself used for communication are not compromised. Or subverted.
It's common knowledge that group chats are not E2E - there is one encryption context from a user to the servers, and another context from the server to each member of the group chat. Bog standard transport layer security, in other words.
However, even if you never used group chats and had E2E on with all your contacts, the traffic analysis ("metadata use") is enough to build associations and clusters. FB doesn't need to know the message contents (although they make use of them when available). You have frequent chats with people who play certain kinds of sports? Fine, for marketing purposes you'll be grouped with people who like those sports. Or if majority of your friends have pets - guess which cohorts you end up as well.
Oh, and if I remember correctly, WA definitely processes your messages locally before sending them: it uses a list of image hashes to prevent sending eg. child exploitation material onwards.
