facebook share button missing on photos

It is no longer possible to share directly from Photos to FaceBook.

Apple removed the integration of social media more than a year ago, when macOS 10.14 Mojave came out. The accounts for social media have been removed from the System Preferences, I think for security reasons.

You now have to use the uploader on Facebook to share photos and videos.

You can only share from Photos to social media sites, if the site offers a sharing extension at the App Store.

See: Share photos using other apps and websites in Photos on Mac - Apple Support

Yes, that's right. You'll have to use the Facebook "Add Photos" feature with your browser. The window will look like this:

I created a special album to make it easy to locate the photos to upload to FB.