A fair point, because Walmart, Target, CVS, Costco, etc all vertically integrate and sell their own (often whitelabeled) products on their shelves, alongside other companies' goods.
In that case, the core product (to other businesses) is shelf space.
So is there a monopoly on shelf space control?
Walmart is huge. But Target's pretty big too (~10% their size by revenue). And they've got bigger competitors in grocery.
But in the Facebook case, where else are you going to buy eyeball time? The closest thing to the scale of Facebook ad space would be if Google put ads on the launcher of Android.
