I am on Pixel 3 XL. I found that Google Photos recognizes the Facebook photo folder (and some others, such as Screenshots) the same as the "Camera" folder because they are within the DCIM folder. Therefore, we cannot tell Google Photos not to backup these folder within Google Photos settings.
The only option I have come across is to put a .nomedia file in the Facebook folder. That does prevent Google Photos from backing it up, but it also prevent me from seeing that folder in any other app. As you know, the Pixel does not include a traditional photo gallery, as they want you to use the google photos app so I use a photo gallery app from the play store. Either way with the .nomedia file in that folder I cannot see the images in that folder anywhere.
Is there any other way to stop Google Photos from backing up this folder without making the images invisible on my device? Google photos should not be treating all folders within DCIM as "Camera" and instead it should be looking specifically for the camera roll folder.
Related post: Change Facebook (and others) photo saving folder