In Facebook, where you type your username, it says "E-mail or phone". Can you login to Facebook using your phone somehow? Would the password be the same as the e-mail?
You need to add your phone number in the
Account settings. Once you did it, you can use your phone number as a user name with the same password you've used with your email or user-name.
Yes, you'd be able to log in with the same password that you'd use with your email, if you used your phone number. According to this Facebook FAQ:
If you signed up for Facebook from your mobile phone, you can use your phone number and the password you created during the sign up process to log in on from a computer or phone.