I have trouble believing that they didn't check any other websites when they were preparing that blog post (and facebook would be the obvious next choice to test), but another reason not to mention facebook may be that not many people would notice the effect of the doubleclick cookie's absence in the ads that they see, but most people who use Like buttons (and facebook embeds, etc) would notice when they suddenly stopped working.
So if people did use that new IE9 blocking feature with the suggested addition of google, and then facebook and all the sites found here were added too...you'd get the theoretical hit to facebook revenue, yes, but you'd get the definite hit of users all thinking your browser is broken because it won't load Facebook anywhere but facebook.com. Imagine those support tickets.
(I believe this kind of interaction is almost the exact reason Mozilla decided not to block third party cookies by default and why Apple decided to add the exceptions in Safari...someone posted a bug earlier with an Apple engineer arguing for the exceptions specifically because of Facebok and Microsoft Live logins, among other things).
edit: the bug was in comments on the ars coverage: https://bugs.webkit.org/show_bug.cgi?id=35824