Can't you already sign up for all of the services independently? I know I can login to instagram with my fb login, but I still had to 'create' the account the first time I did. I could have just created a standalone account if I hadn't had an fb account to start with. Messenger is essentially facebook chat pushed into a separate client, which I had thought they were talking about reincorporating back into the main client, I don't really see why should allow a separate login for that.