According to this article the common shares were worth ~$120 in 2019. Preferred will be different from common, but this would mean the investors saw no gains from 2015 to 2019.

Either some info is incorrect somewhere, or there was some split or valuation indeed didn't change much in 4 years.

https://www.vox.com/2019/3/19/18272274/airbnb-valuation-comm...