There's an enormous challenge in Uber's future, and that's driverless cars. If Google, GM or Baidu develop the technology first, Uber could find its growth seriously curtailed or its service priced out of the market altogether. That's why it's betting so heavily on developing self-driving car technology in house. This is an existential dilemma for Uber. I wonder how investors will value the company in the inevitable IPO if there is still ambiguity around this question?
Airbnb seems to have a cleaner, clearer future. There are very few direct global competitors, and Airbnb is even making progress in China, viz a viz local competitors like Xiaozhu. It might end up being one of the few Silicon Valley companies to have an unencumbered global network. The question then becomes whether it can successfully branch out into other areas, like offering travelers things to do during trips, booking plane fares, and facilitating restaurant reservations. The future success of its "Trips" platform launched in 2016 will tell us whether Airbnb can truly become a travel giant.
Overall I'm pretty optimistic about these companies and admire their CEOs. They have seasoned advisers around them and have the money and momentum to hire the best executives. The next few years should be interesting to watch.