Facebook for Android is now being Updated every week with bug fixes and New features, You can Now try out the latest App version of Facebook For Android version 126.96.36.199.1 Alpha Build with Android 5.0 Lollipop API 21 support. In this Version users can now log into their accounts faster without typing their password with "Fast log-in". While you still stay signed in by default as before, if you are logged out for some reason, you can get back to your own account by just tapping on your profile picture.
This is especially useful for those times when a friend needs to sign in on your device or if you have multiple accounts. Of course, before enabling this feature, you will want to consider that it's just as easy for someone else to log into your account as it is for you. You can add a 4-digit passcode to balance the user-friendliness with a modicum of security.
After downloading the latest Alpha build, Facebook suggests turning on automatic updates as it plans on updating the app a few times a week. Doing so will ensure you're always running the latest version. The APK files provided below is the official APK from Facebook ALPHA program and hasn't been modified. The first one is for devices running Android 4.0 or later, the second link for the devices running Android 5.0+ lollipop.