The real monopoly problems in our economy are not the firms that push up some very particular concentration indices, rather they are the small, local monopolies, hospitals, and the public education system. Here is a new investigation (AEA gate) from Sharat Ganapati, you will note that the bold emphasis has been added by yours truly:
That is from the new issue of American Economic Journal: Microeconomics. Rooftops! Other research has pointed in the same direction. Pennsylvania, Ave.: please do not split up America’s best and most productive firms.