Hilton (HLT - Get Report) and Marriott (MAR - Get Report) are already feeling the heat from Airbnb's entrance into the lodging market. And it's going to get hotter for the hotel giants, says Leigh Gallagher, author of The Airbnb Story.

Airbnb, now valued at around $30 billion, is expected to grow its revenue to $2.8 billion in 2017 and $8.5 billion in 2020. The company has roughly 3 million listings in 191 countries, 80% of them outside North America.