I had problems to start with, using tapatalk 2. Entered my info real slow and deliberate, make sure you have the box checked so it shows the password. Sometimes it takes a few times. Now I don't have any problems, I use tapatalk 2 for many other forums as well. The info is case sensitive as said before. Maybe restart the phone after any installs.
I went back and re-read your post, I don't use facebook to log into anything, except facebook. I think that's the problem.