Facebook App Won't Update
Edit: this appears to be a concern that Facebook are aware of. It's a concern with the Facebook application, and not a generalised Play Shop concern.
This is what worked for me at the very least.
- Settings > Applications > All > Google Play Shop, Clear data and also Clear cache
- Settings > Apps > All > Google Play Provider > Manage room, Clear all information and Clear cache
It doesn't hurt if you reboot your phone before you attempt updating facebook again. As well as, the install was SLOW. Just a minimum of 4-5 mins. However, it did job. Otherwise attempt to uninstall it, or merely wait on a solution.