Every driver I have seen is driving for both services and would add a third if someone told them about it and it payed equitably, so I think all the work uber and Lyft have done to make a market can be leveraged by third party competitors if they start dialing up prices to become more profitable. And I think they will need to do that to meet their valuations.
This isn’t like Facebook where a larger network acts like a moat keeping competitors out. Instead, I think it actively makes a market for competitors- the friction for drivers is pretty small, and I don’t know the friction involved for getting users to download another app but I imagine it’s pretty low. Because the drivers are the same it’s a commodity, and switching between apps costs me nothing.
Disclaimer: I probably don’t know what I am talking about.