Drivers often list themselves on both Uber and e.g. Lyft in the same way riders use both apps. So there are no network effects in play. All the drivers need to do is switch the signs on their windows.

It's the reason Uber is taking such extreme measures to scare "it's" drivers into not simultaneously listing themselves on competing apps. See: http://money.cnn.com/2014/08/04/technology/uber-lyft/