Imagine a not-too-distant future in which trustbusters force Facebook to sell off Instagram and WhatsApp. Imagine a time when Amazon’s cloud and delivery services are so dominant the company is broken up like AT&T . Imagine Google’s search or YouTube becoming regulated monopolies, like electricity and water.

Facebook，Google parent Alphabet and Amazon are enjoying profit margins, market dominance and clout that, according to economists and historians, suggest they’re developing into a new category of monopolists. They may not yet be ripe for such extreme regulatory action, but as they consolidate control of their markets, negative consequences for innovation and competition are becoming evident.

Together, Google and Facebook take in 73% of U.S. digital advertising. It may not be something you think about often, but that success rests largely on the fact that both have spent so much money building data centers and filling them with hardware and software designed by an elite, in-demand set of engineers. In this way they resemble the telegraph giants, with investments in physical infrastructure so large no upstart could match them.

Amazon, in its sprawl and ambition, illustrates what monopolies look like in their early days, says Kim Wang, an assistant professor of strategy and international business at Suffolk University’s Sawyer Business School. Amazon seems determined to translate its dominance in cloud computing and online retail into dominance in physical retail, delivery of goods, voice-based computing and a half dozen other industries.

Amazon already accounts for 44% of U.S. e-commerce sales, and is showing rapid growth in categories where it previously foundered, like luxury goods and food. It’s convinced former competitors to get on board as partners, is vertically integrating everything from ordering to delivery—and could someday add manufacturing to the mix.

While Apple may be hoovering up the lion’s share of the mobile industry’s profits, the company is hardly a monopoly by measure of overall market share, say experts.

A “network effect” is when a product becomes more useful as more and more people use it—for example a fax machine or Facebook. For Apple, the size of its customer base attracts developers who in turn make the iPhone and iPad more valuable.

In every monopoly-dominated industry in history, whether it was oil, railroads, steel or utilities, even the most avaricious competitors took decades to consolidate their hold on markets. Even at today’s faster pace, it’s probably still early days for tech giants.