Can I safely delete .facebook_cache folder if I don't have a Facebook app installed?
Quoting from Mathew Read's answer here


  Cache is temporary files. One example might be thumbnails for contacts in a social media app. These can be cleared without any major effect — the app can just download them again when it needs to — and if space is low the Android OS may remove cache files itself.


You don't even have the app installed , so it can do no harm

The reason for it being of likely the Google Backup, since you never installed the app 

You can also use the SD maid app mentioned here, which I find very effective in removing corpses and cleaning system How to clean up related files from deleted apps?