> Facebook et al know mostly about how you externalise your identity towards other people. The data they store would be well structured, but would also be limited, mostly to what you would tell your closest friends.

100% bullshit, for Facebook at least. They have user-tracking elements all over third party websites just like google does (maybe not quite as many), and if you have any of their apps on your phone (including things they own like instagram and whatsapp) then they have access to almost the same data that google does.