This only applies to Windows 8, 8.1, WP 7 and 8. It's basically the Facebook connection to your Microsoft Account. The People app, and other fb web-based Microsoft services like fb chat on Outlook and Calendar use this connection. It's quite old since it's been there since WP7. The major downfall with this method is that everything on your fb feed and activity goes through Microsoft servers, so if MS servers fail or goes on maintenance you won't be able to get your feed, so fb on your devices goes kaput.
Unlike on WP 8.1, the feed come from the app themselves, which is a smart move actually. So if you remove facebook on your WP 8.1, you'll totally get rid of facebook on your phone, except that annoying facebook icon on the people hub tile. Twitter and LinkedIn already do this on WP 8.1, unfortunately can't say the same on the desktop (8.1) variant.
To be fair it was really nice to have that fb/Hotmail chat integration in the Messaging hub in WP7. Really just waiting for this kind to happen with 8.1 or 10.