Last month, Microsoft announced that they had partnered with Facebook to bring Facebook and Instagram as Universal Windows apps.
The Facebook universal app was released to Windows 10 mobile phones recently but wasn't available on Windows 10 PC.
Well, today you can now download the Windows 10 version in the Windows Store but there's one problem, you can't login. If you try to, you get a error code 404 when you do. So it's not quite ready yet. But we expect this bug to be fixed soon.
You can download it from the Windows 10 Store