Early this morning, some users had reportedly found the expected universal Windows 10 app for Facebook. Although we have that app for mobile the version for PC is reportedly coming in December (along with Uber and Instagram).
Update: Facebook has reached out to Windows Central and have confirmed that the app is authentic:
"We're testing Facebook for Windows Desktop, a new app for Windows 10 computers. We look forward to rolling this out more broadly next year."
Alternatively, @NotCassim, who writes for FraWin and is well versed in Microsoft, broke down the app with Florian B. to find iOS components in it. These files imply this could be an Islandwood bridge project making it a port from iOS that has been recompiled for Windows 10. Adding to the mystery Zac Bowden from WinBeta noted the screenshots in the Store come from Michael Boswell, who is a Facebook engineer. We have heard from our sources that Project Islandwood is working well, Astoria however, is a different matter (more on that later).
Update 2: New evidence has come forward and we do not believe this is an iOS port. However, in this early beta stage, they are likely re-using iOS graphics as placeholders.
Either way, since you cannot log in and use the app, this is a bit less interesting. After all, we know the app is coming so it is just a matter of time. We'll wait for Microsoft and Facebook to make an official announcement, but it should not be too much longer. As a reminder, from the October Microsoft event Facebook went on record that they are supporting the new OS:
"Facebook is all in on Windows 10 – we're excited to build Universal Windows Apps for Facebook, Messenger, and Instagram." -Mark Zuckerberg
As far as trying to log in if you download this app, just have a replacement password ready in case you notice any suspicious activity.
Thanks, @NotCassim, for the info!