Earlier today, Microsoft announced that the new Facebook, Messenger, and Instagram apps for Windows 10 would become available in the Windows Store. They are now showing up and available to download from the links at the bottom of this page.
However, make sure to double check that the store page says Built for Windows 10 so know you are downloading the latest version. If you see the logo below, then you're seeing the newest Facebook app.
The new apps bring parity to all of the features you are familiar with on other platforms. WinBeta editor Zac detailed the new features this morning, which include things like support for GIFs, group conversations, stickers, and more. Currently, the Facebook and Messenger apps are just for Desktop PCs but they will be making their way to Mobile eventually. And Instagram is only available on Windows 10 Mobile.
It's nice to finally see full-featured version of some of the most downloaded apps in the world coming to the Universal Windows Platform. There still might be hope that the app gap will be a thing of the past someday.