Ask HN: Getting better user engagement

Our startup launched our app last Friday and as a promo we gave away $5 in credits to everyone. We have had a good response of people downloading and signing up with the app, but only a few people have actually taken advantage of the deal.

Any suggestions on getting a better response from our users?

For reference our app is https://itunes.apple.com/us/app/flippo-student-powered-delivery/id836031729?ls=1&mt=8 and is currently open to Pitt and CMU students.