Facebook Won't Update
Edit: this appears to be a problem that Facebook understand. It's a problem with the Facebook application, and not a generalised Play Store issue.
This is what worked for me a minimum of.
- Settings > Applications > All > Google Play Shop, Clear data and Clear cache
- Settings > Apps > All > Google Play Provider > Manage area, Clear all information and Clear cache
It doesn't harmed if you reactivate your phone before you attempt upgrading facebook once again. And also, the install was SLOW. Simply at least 4-5 mins. Yet, it did job. If not try to uninstall it, or merely wait for a fix.