The reason that I asked about root is that the behavior you describe sounds a lot like what happens when you are rooted and move and app from where they are normally installed, in the /data/apps, to the /system/apps. This happens with apps that are built-in to the ROM, like Google Maps, when there are updates in the market but there is a base install that was built-in. The thing is that the Facebook apps is not built-in to the stock Eris build - or, at least I don't think it is.
If you want to root, there are a lot of us ready to help you through the process, when you're ready. Look in the All Things Root subforum.