Hotel stocks are on the rise this year, but investors may be discounting the long-term impact of home-sharing.

t’s been a surprisingly strong year for hotels.

Both Marriott International (NASDAQ:MAR) and Hilton Worldwide Holdings (NYSE:HLT) have outperformed the S&P 500 by significant margins; Marriott shares have gained 30%. Meanwhile, online travel agencies continue to thrive. Both Priceline Group (NASDAQ:PCLN) and Expedia (NASDAQ:EXPE) have both climbed more than 25%.

Despite those promising performances, there’s a threat rising unlike any the industry has ever faced before as Airbnb upends the primacy of hotels.

The home-sharing start-up that was founded in 2008 is now worth $31 billion, based on a funding round earlier this year. That puts it ahead of Hilton, InterContinental Hotels (NYSE:IHG)Hyatt Hotels (NYSE:H) — but behind Marriott — following its acquisition of Starwood Hotels last year. Airbnb now has more rooms available than the five biggest hotel brands combined.

And Airbnb’s barnstorming is far from over. Revenue jumped more than 80% last year, and the company turned profitable in the second half of 2016 and expects to turn a profit again this year. The travel site is expanding into ancillary businesses like experiences, allowing “hosts” to offer tours or other kinds of activities designed for tourists, and is planning to allow flight bookings on its platform.