The most secure way to check for app availability is still through the Microsoft Store. If your Windows 8.1 system is up to date, you may still be able to find a version of the official Facebook app there.
If you decide to use a dedicated app instead of a standard browser, follow these steps: Facebook for Windows 10 - FileHippo download facebook for pc windows 8 filehippo
Sites like FileHippo provide version histories, which can be helpful if you are looking for older, lightweight installers specifically built for legacy systems. How to Install Facebook on Windows 8 The most secure way to check for app