You'd be surprised. Since switching is so low, you can pay your way to acquiring all the supply and demand to kick something off, hence all the ridesharing companies in NYC: https://finance.yahoo.com/news/ubers-competitors-in-nyc-are-...
If you subsidize driving and subsidize riding significantly, people will sign up. A $40 bonus covers several rides for a customer so they'll hold onto the app at least until they've used their initial free offering.