This article is talking about Windows 8, not Windows Phone 8.
And Windows Phone has had OS level integration with Facebook from day one (i.e. the launch of Windows Phone 7).
And Windows 8 has sold 4 million copies as of the last figures I saw.
Facebook doesn't typically do OS-level integration on any OS, and clients are only really useful on mobile devices (desktop-spec web browsers and connections are very much good enough for Facebook).