There's still a network effect; it's just a duopoly. The third outfit trying to make this work can't, because they don't have anyone with their app on the phone, because they don't have any drivers, and they don't have any drivers because nobody wants to drive for them because they don't have any customers.

Seems like it would be trivial for someone to do exactly what Uber has done - pick a location and undercut everyone on price using predatory pricing. People will install a new app for a buck or two savings, and drivers will install a new app to get a 5% raise.