The market, search for facebook.
I installed it this morning, also told me it would replace current version, and you have option to uninstall update from apps management.
What would be the better question is how to determine the version of the originally installed app, as I looked around before trying the download.
In addition one would think that you would get a update available message, even if it were installed by default ... Maybe facebook didnt include something in its upload to the marketplace for that, when google Maps released an update, all of us knew it and it began as a default app on the Droid ...
Load it, like it, or not, you can uninstall ...