Depending on the content of the ads, I could easily believe that anyone who is remotely the market for even the occasional Uber ride already has the app on their phone and, if they want a ride will either use Uber, whichever ride-share is cheapest, Lyft if it's available because they don't like Uber, or a taxi/limo in situations where that's more convenient.

I guess you can offer promotions but then you're giving money away.