[yellow_postit]

Ad companies like Google tend to be winner take most markets. Thats why you see it referred to as the duopoly of Google and FB, together they have ~60% of the digital ad market.

[joshuamorton]

This doesn't address the question:

Why didn't something pop up and eat up the success left in the wake of a market Google left, like reader or inbox or wave?

[ryukafalz]

I would guess because those products weren't profitable on their own. They were propped up by Google's massive advertising business.