Therefore, according to statists, it must be an anti-competitive "monopoly." The Federal Trade Commission and the Justice Department, the argument goes, need to take judicial action against Amazon to "weaken it" by breaking it into independent competitive parts. In this way, America's "free-enterprise" system will be strengthened.

It's all pure, unadulterated economic nonsense, oftentimes driven by envy and covetousness.  

In a free-market economy, a company gets big and successful by satisfying consumers. If it produces goods or services that consumers like, it makes money. Amazon has clearly done that. Beginning as a book seller, Amazon now sells everything under the sun. The reason it is so big and successful is that it has satisfied consumers.

In a free society, a company has the right to become as big and successful as it wants. In the absence of fraud, a company's bigness and success is none of the government's business. This includes the right to merge with other companies, thereby becoming even bigger. After all, we are talking about private property. A person's private property is his. As such, he has the right to sell his business to whomever he wants, including a larger firm, even if the sale means a smaller number of competitors in the marketplace. 

Statists claim that if enterprises are free of government control and regulation, a few businesses will get bigger and bigger and finally "monopolize" major sectors of the economy.

Really?

Then how do they explain the fact that the most of the top 50 companies in the United States in the 1960s are no longer in the top 50 today? If big companies just keep getting bigger and more powerful, then those top 50 companies in the 1960s should be gigantic enterprises today. But they're not.

The reason is consumer sovereignty. By their purchases, consumers decide which companies are going to be big and prosperous. Those top 50 companies in the 1960s were unable to continue satisfying consumers. Other businesses induced their customers to shift to the new companies.