That permanence is a bit of a myth. Bit rot is as real as physical one. At least four cloud storages (Bitcasa, hubiC, Ubuntu One, Cyphertite) I’ve used in the past are gone.
User agreements change constantly, engineers make mistake, firms get liquidated and data might get sold, and most importantly, as a former employee of social media firms, what the firms say about the user privacy publicly is very different inside.