Does anyone know what they do share besides phone number? And what could they share? I know end to end message encryption is a thing, but can the app still supply your messages or analysis of them to FB?
They can presumably not share the content of the messages, but they could share the metadata around them. Who you are connected to, how often you contact them, what time of day, potentially some location information, how long you spend in the app...
Facebook, for example, can probably establish who my wife is, even if I don't explicitly put her in my profile, and who effectively are my affinity groups (friends/family/etc).