In an excerpt from The Upstarts: How Uber, Airbnb, and the Killer Companies of the New Silicon Valley Are Changing the World published by Bloomberg on Thursday, author Brad Stone examines how Uber CEO Travis Kalanick and Airbnb chief executive Brian Chesky created (and dominated) the sharing economy.

The two men rose to prominence together — Airbnb was founded in 2008, Uber in 2009 — using the ubiquity of the smartphone to build on-demand empires, Kalanick with ride-sharing and Chesky with home-sharing. Both founders’ missions quickly took off. But as Uber and Airbnb grew, they repeatedly clashed with regulators, competitors, and even entire governments.

The duo had plenty to bond over, so it’s not shocking that the founders “struck a sporadic friendship” over the years. According to Stone, the founders would meet up for dinner every few months in San Francisco and swap tales of their latest regulatory nightmares. Chesky says they learned a lot by watching each other; over the years, both companies have faced similar challenges regarding regulations, but also security and privacy.

Related: How Airbnb Found a Mission—and a Brand

Stone spoke with an Airbnb executive who says many employees knew when one of these dinners took place because, “Brian would come back saying, ‘We have to be tougher!’ and Travis would come back saying, ‘We have to be nicer!’”

Whatever they learned from one another, it appears to be working. Airbnb is valued at $30 billion and Uber at a whopping $69 billion. Airbnb is one of the largest room rental marketplaces, and yet it doesn’t own any hotels. Uber is one of the world’s largest car services, yet it owns virtually no vehicles.

“They’ve attained these heights, and a combined worth of $99 billion, despite owning little in the way of physical assets,” Stone writes.