The app is both on the web and on mobile (iOS and Android app). When you use it on the web, we advertise the mobile version, with the option for the user to give his phone number to receive an SMS on his phone with the install link. It serves as a reminder and ease the experience of finding us on the app store with a way better conversion rate than an email.
Slick - love that use case. Totally one we didn't expect, by the way, but has ended up being super effective.
Super easy to implement and test on GAE too with the Tasks API. The testbed makes an async implementation with full coverage half an hour of work. Big fan of the supplementary services Google provides here.