I would say it's the people visiting google that's the gold vein - advertising is the pick-axe. You don't make money with ads if no one visits. So in a sense making more and more products and reasons to visit is the gold vein.

Otherwise just saying advertising is a gold vein would mean people that put up a blank page with a single ad on it and no reason to visit would definitely not be a gold vein. So as others mentioned, its the Youtube and Gmails level products.