The mainstream media are self-reinforcing. Auferre, trucidare, rapere, falsis nominibus imperium; atque, ubi solitudinem faciunt, pacem appellant.
Many social phenomenon are most easily described in this way. Generally, the easiest way to find out who is is a member of a subcommunity is to ask other members of that subcommunities.
And as Migeru said, its a combination of that influence among the rest of the mainstream media - and thus general public discourse, which they catalyse - and audience numbers - some influential magazines are more "insider media", being read by those that make the news, but not really bringing these news to the public. Auferre, trucidare, rapere, falsis nominibus imperium; atque, ubi solitudinem faciunt, pacem appellant.
Wikipedia: I know it when I see it
Justice Potter Stewart used the phrase in his concurring opinion in Jacobellis v. Ohio 378 U.S. 184 (1964). He wrote: "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that." (emphasis added)
"I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that." (emphasis added)
