Why does Facebook not let me log in?
Facebook users who are unable to log in may be having a cache or cookies problem. To fix these type issues, users should clear the cache and cookies on the computer or mobile device, enter "m.facebook.com" as a URL rather than using previously established bookmarks or links, then log in as usual.
Why is Facebook not working on my Android phone?
Check for app updates
If problems persist, you want to make sure you're running the latest version of the app. Head over to the Google Play Store (or any app store you use) and check for any available updates. Get the latest version and check if Facebook is working.