A monopoly is someone who owns an entire market. In the history of america, the only monopolies that have existed were government granted ones, like the US Postal Service. The government passed a law prohibiting the offering of first class mail. At the time they did this, there was a thriving and growing and diverse mail delivery system made up of thousands of small businesses and private contractors.

Standard Oil was never a monopoly, and never had monopoly pricing power. Standard oil was only ever able to get large because they quickly and constantly drove down the price of gasoline.

There is this tendency for americans to call businesses that are not monopolies under the definition to be monopolies to justify using violence against them (it is violence when a company is forcibly broken up or forced to not engage in "anti-competitive" (by which they really mean "competitive") activities.)

The reality is, none of these businesses were monopolies, and as much as I hate microsoft, they weren't a monopoly. Existing laws would have been sufficient for enforcement against microsoft for their crimes, which included fraud and misrepresentation. But the "anti-trust" laws are all about preserving government power and are used only against businesses that become big enough to wield influence and thus attract opponents in government... usually opponents in government in the pocket of competing businesses.

The US would be a better country and have a more robust economy, if Standard Oil had never been broken up. This and the "anti-trust" movement is a triumph of socialism over capitalism.