I use Facebook app to browse through Facebook, unlike others who are too concerned over app's permissions & use browsers.
But there's this thing about the app which you would not find in any browser. In browser if you even type a letter wrong you will be asked to login again.
But on Facebook app even if you type in wrongly the username, the app will auto fill the username that you're trying to write by matching each letter to the username in their database. And then you just have to enter the password.
This could mean many things to many people, for some it's a convenience of typing the username correctly.. for some who know the password or others it will be an easy hack.