I just downloaded the Stack Exchange app from Play Store and when I tried to login there was no option to login with Facebook, am I missing something? Is it connected with OpenID?
I'm guessing your login screen looks like this:
But if you have the official Facebook app installed, the login screen looks like this:
Per Is Facebook an OpenID provider?, Facebook is not an OpenID provider. So you'll need to install the official Facebook app to login via Facebook in the Stack Exchange app.