Market downloaded and installed apps are in the /system/app folder.
You will also find the ones that come with your android build but unable to delete those.
Unless you convert your system.sqsh file into an system.ext2 file. If you wish to do this you will need the Stay Safe App and follow the guide
here. Not recommended if you run the CAB or startup tool updater.
*this will NOT fix the facebook app from updating/installing fresh with XAndroid builds.
You can also break your android if you change/delete the wrong files! Experts only suggested but feel free to learn!