3. Airbnb

Rewriting the book on bookings.

Founders: Nathan Blecharczyk, Brian Chesky (CEO), Joe Gebbia
Launched: 2008
Headquarters: San Francisco
$3.4 billion (PitchBook)
Valuation: $31 billion (PitchBook)
Key technologies:
Artificial intelligence, machine learning
Hotels, travel

George Kavallines | CNBC

As it enters its second decade, Airbnb is proving that it's more than an online marketplace for home sharing. In 2016 it launched Experiences, where guests can explore a location through unique activities run by locals. Among the offerings: hiking in the woods with wolves in Seattle or learning how to make pasta in the home of two chefs in Florence, Italy.

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Co-founder and CEO Brian Chesky is especially bullish on the potential for this segment of the business. He figures as younger people increasingly migrate to cities, more of their disposable income will be spent on experiences rather than possessions. Right now the company has about 3,000 experience hosts, with thousands more waiting to be rolled out. Chesky recently told Fortune that this segment of the business is growing 13 times faster than its home-sharing business was growing at the same age.

Not that the home-sharing business is anything to sneeze at. The San Francisco-based giant is now active in 81,000 cities in 191 countries and has over 4.5 million listings on its site — including 3,000 castles and 1,400 treehouses. It recently rolled out tiers for higher-end customers, called Airbnb Plus and Beyond by Airbnb. The homes and apartments listed here have been verified for comfort and quality and feature amenities such as faster Wi-Fi and upscale appliances.

Of course, there have been rumblings about the impact the company has on the social fabric of wherever it operates. Over the years, it has angered regulators in San Francisco and New York City, who question whether the rentals are getting taxed properly. There also have been complaints that Airbnb hosts were racially discriminating against African-Americans and other minority groups by declining their requests for bookings after looking at their profile photos.

None of this has put a damper on the company's ability to attract investors. To date it's raised $4.4 billion from Andreessen Horowitz and Sequoia, to name a few. The company reportedly earned $100 million last year, making it the first full year it's turned a profit. And as for that long-awaited IPO, there's still no word on when — or if — the company will be going public anytime soon.