Hello simonfromseaview,
Welcome to Apple Support Communities. It sounds like you're having trouble accessing Facebook on your Mac after upgrading to macOS Big Sur. We are happy to share information that may help.
First, start up in safe mode to see if the issue occurs in safe mode: How to use safe mode on your Mac.
You may also want to test this issue in another user account on your Mac. If you don't have another user account set up, you can create one using the steps here: Set up users, guests, and groups on Mac
Finally, if you're having trouble loading the Facebook webpage, try the steps provided in this article: If Safari doesn't load a page or webpage items are missing.
Take care.