Why Won't My Facebook Update
Edit: this appears to be an issue that Facebook know. It's a concern with the Facebook app, and also not a generalised Play Shop problem.
This is what benefited me at least.
- Settings > Applications > All > Google Play Store, Clear data and also Clear cache
- Settings > Apps > All > Google Play Solutions > Manage space, Clear all information as well as Clear cache
It doesn't hurt if you reboot your phone before you try upgrading facebook again. And, the install was SLOW. Just at least 4-5 minutes. However, it did job. Otherwise try to uninstall it, or simply wait on a solution.