Which Facebook URL should I build backlinks to?

If you are targeting French audience then I'd say go for fr-fr url.
 
Have you checked if they're both returning the same headers and both are indexable and not blocked in robots.txt or anything? I think it would be strange of Facebook to allow the same page indexed many times only with different interface languages.
 
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features and essential functions on BlackHatWorld and other forums. These functions are unrelated to ads, such as internal links and images. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock