That's kind of the "gotcha" question they ask you when getting a passport in an overseas office: "have you ever been affiliated with a terrorist organization or performed terrorist acts in another country?" At face value the question is ridiculous, but they put it there just to make sure they've got you on record lying to a government official if you are ever found out to have lied.

Now, the problem with the Facebook/Twitter thing is that the really bad guys are not going to be stupid enough to have Facebook and Twitter accounts where they say they'll be committing terrorist acts, and they won't worry about being caught lying later on. So all in all, this sounds like posturing for posturing sake.