Got this myself on my phone, and it just refused to update. Here’s what worked for me.
Edit: this seems to be an issue that Facebook are aware of. It’s an issue with the Facebook app, and not a generalised Play Store issue.
This is what worked for me at least.
- Settings > Apps > All > Google Play Store, Clear data and Clear cache
- Settings > Apps > All > Google Play Services > Manage space, Clear all data and Clear cache
It doesn’t hurt if you restart your phone before you try updating facebook again. And, the install was SLOW. Just at least 4-5 minutes. But, it did work. If not try to uninstall it, or simply wait for a fix.
For more troubleshooting suggestions, I recommend that you read THIS