Facebook App Won't Update
Edit: this appears to be an issue that Facebook know. It's a concern with the Facebook application, and also not a generalised Play Store problem.
This is what worked for me at least.
- Settings > Applications > All > Google Play Shop, Clear data and Clear cache
- Settings > Apps > All > Google Play Provider > Manage space, Clear all information as well as Clear cache
It does not hurt if you restart your phone prior to you try upgrading facebook once again. And, the install was SLOW. Just at least 4-5 mins. But, it did work. Otherwise attempt to uninstall it, or simply await a solution.