Just as the name suggests, if you’re currently running Windows 8.1 RTM, the updated first-party (and some surprise third-party) apps have already been released to the Store in anticipation of the General Availability release arriving in about six hours. This includes an update to Mail/Calendar/People, Skype, Xbox Music, Xbox Video, Bing apps, Windows Calculator, Windows Reading List, and even Adobe Reader Touch. The first-party apps accounted for roughly 19 or so updates in all.
Update: The official Facebook app appears to finally be making its way into the Store as well. Out of three PCs, I only see it appearing on one so far. But once you get it installed, you should be able to get it on any PC by syncing your app licenses and then going to the “Your apps” section of the Store.