Develope your personal
The app is a constant reminder of your company that they will see anytime they cross the menu.
Whenever one gets to check e-mail, google something or order pizza, your app will blink through their eyes. It is a more psycological approach, but very effective.
Besides, it will save you a lot of money, time and energy owning an app, rather then investing in advertise that will waste paper, involve much more people and cost more.
Everyone owns a smartphone nowadays, so why not benefit from that and make it useful?