"Clear data" does not clear login details I was under the impression that "clear data" clears everything, but apparently it does not. Tested with a few apps, including Facebook. 

I stopped the app, cleared cache, cleared data, opened it - and by some miracle it still remembers my login details! How does it do it? How can I disable this?
You probably also need to remove the program account details from Settings -> Accounts and Backup -> Accounts

It is probably there that the account details are being saved. Mainly to allow you to safely clear data (to reset the program) without the hassle of having to log back in.