Four firms —Facebook, Google, Amazon and Apple — now form the de facto roster of Big Tech, thanks to Congress' decision to interrogate their CEOs together at a landmark antitrust hearing Monday. (Sorry, Microsoft, but maybe it's for the best.)

The big four share enormous power, massive resources, high ideals and, more recently, troubled public images. But there are enormous differences among them, too — and their leaders will be leaning on those contrasts as lawmakers grill them.

Why it matters: These differences will shape how each firm fares in the court of public opinion and the labyrinth of regulatory action as they try to counter lawmakers' charges of monopolistic practices, privacy violations, censorship, and tolerance of misinformation.

What they do:

• Facebook's mission is to "bring the world closer together" through its social network: it connects people with other people.
• Google, whose mission is "to organize the world's information and make it universally accessible and useful," connects people with information.
• Amazon's aim is "to continually raise the bar of the customer experience." Let's just say it connects people with goods.
• Apple's mission, "to make great products," might be described as giving people the best devices to make all of the above possible.

How they make money:

• Both Facebook and Google sell ads. Together, they're predicted to collect roughly 52% of all U.S. digital ad dollars this year (per eMarketer).
• Amazon is a retailer that also runs a marketplace for other merchants. It also has a huge cloud services business ($75 billion in revenue for the first quarter of 2020).
• Apple sells hardware. It's also building revenue from services — more than $13 billion in the most recent reported quarter, making that line bigger than its Mac and iPad businesses.

Who's in charge:

• Zuckerberg founded Facebook and retains a controlling majority of voting shares, which makes him effectively an absolute ruler.
• Bezos founded Amazon and has run it from the start, though his substantial stake does not come with voting control of the company.
• Cook and Pichai both succeeded powerful founders (Steve Jobs and Larry Page) in the CEO chair, and though there's no sign that either of them faces a challenge, they are much more answerable to their boards as a result.

Where they're based:

• Facebook, Google, and Apple are all headquartered within miles of each other a half-hour south of San Francisco.
• Amazon, like Microsoft, is based in Seattle, but it gets lumped in with Silicon Valley nonetheless — at least as a state of mind.

How they're structured:

• Facebook acquired Instagram in 2012 and WhatsApp in 2014 and, though it has moved to integrate them technically, it still operates both as distinct services. That gives lawmakers a potential way to think about breaking up the company.
• Google's acquisition of DoubleClick in 2007 set up its dominant position in the digital-ad market. YouTube, its vast video platform, also operates independently.
• Amazon's hugely successful Web Services cloud service operates relatively separately from the company's main business, and some critics have called for the two to be separated.
• Apple has rarely grown through big acquisitions and offers regulators few obvious fissures to cleave. If they wanted to consider somehow splitting it up, they'd likely explore breaking the App Store off from the iPhone's operating system and Apple's control.

Yes, but: The fact that there are four tech superpowers to begin with suggests there's still plenty of competition that opponents of antitrust action can cite.

These companies are all in one another's businesses.

• Apple and Google offer competing smartphone operating systems (iOS and Android).
• Facebook and Google's huge ad businesses are in competition with each other, and Amazon competes here too, with its roughly 10 percent share of the digital ad market growing.
• Amazon Web Services faces competition from Google as well as Microsoft, IBM, Oracle and smaller players.
• Google and Amazon both make devices that compete with Apple, and even Facebook has gotten into the act (with Portal and Oculus).
• Each company has deep investments in AI products and research, and each has a voice assistant (Facebook's long-rumored entrant is still in development).
• Microsoft remains a strong competitor in many areas, too.

Our thought bubble: Antitrust is all about defining markets. You can't have a monopoly unless it's clear what the monopoly is on.

• In tech's big previous antitrust epics, cases against IBM and Microsoft, the government took aim at single dominant firms, and labored mightily to make its case.
• Going after four companies at once will make the job even harder.

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