Facebook Does Not Update
Edit: this appears to be a problem that Facebook understand. It's a concern with the Facebook app, and also not a generalised Play Shop issue.
This is what benefited me at least.
- Settings > Apps > All > Google Play Store, Clear data and Clear cache
- Settings > Apps > All > Google Play Services > Manage room, Clear all information as well as Clear cache
It doesn't hurt if you restart your phone before you attempt updating facebook once more. And also, the mount was SLOW. Just at least 4-5 mins. But, it did work. If not try to uninstall it, or just await a solution.