If you try to log in to Facebook using the Facebook Android SDK, and the official Facebook app is also installed on the handset, you may receive the following error: Login Failed: invalid_key.
To avoid this error, you may use the following lines to log in: (it uses the FB dialog to log in, not the installed app)

mFacebook.authorize(
activity,
PERMS_NEEDED,
Facebook.FORCE_DIALOG_AUTH, // avoids SSO
new LoginDialogListener(r));

(Source)

However, if you wish to use the installed FB app to log in an authorize, you should follow the steps presented this site.