Does this happen only when uploading to Facebook? Do you upload to Onedrive, any other offline storage? I know the ocassional device will get warm to the touch when doing "heavy jobs", like uploading data or playing graphic-heavy games. It seems to be a small percentage of devices across all models, smart phone makers, and OS's.
If you do not have problems with any other activity, I would send a messge to the developers (in this case Microsoft) and see what kind of response you get. You can do that by going to the Marketplace online on your computer, search for the Facebook app, and then click on the developer. This way, it will directed to the correct people.