Because it's an erroneous analogy? FB's "content" is created by it's users. It is a platform for people to type whatever they want to share with their "friends" and they make their money via advertising and collecting and dissecting your personal data. I fail to see how that is 1:1 with television networks who, you know, actually create content.
"Why don't we call television networks advertising companies"
Seems a decent analogy, most network television and cable shows are glorified advertising. It's not a brag of any sort to see the lack of meaningful content in the glut of adverts, product placements, artificial "social" engagements.
That said, people do (myself included) lump Facebook into roughly the same category as network television and phone book salesmen.