As root in the terminal, type:

dpkg -l | grep -i facebook

See what names come back. One will be the pre-installed app, and the other will correspond with the one you manually loaded.

They will have different names since they would not both install if they had the same name.

It's likely the the one with a lower version number is the one you side loaded since the official FB app recieved an update recently, but I'm just guessing since I don't know what you installed or where you got it from.

Either way, take note of the name of the offending package and type:

dpkg -r <type offending facebook package name here>