If it cannot be uninstalled, it is part of the "bloatware" that came on your firmware. You won't be able to uninstall it unless you gain "root" access to your phone. It's just as effective and far less risky to just disable the app.
What you do want to do is:
1. Uninstall all updates to it (so you are back to running the old version that is on your firmware image)
2. Go into Settings / Applications / Application Management, find the Facebook app, and do the following:
- CLEAR DATA
- FORCE STOP
- DISABLE
That will keep the Play Store from trying to update it, and keep it from being able to run at all. It's just as good as uninstalling it, without the risk of rooting your phone and trying to rip it out manually. The software is still present on the firmware image of your phone, but it is not allowed to run.