ZocDoc knows them all

Solution

My basic idea was to allude to the fact that it's often elementary to know the right people to get what you want. You can hardly know all the doctors in a city and impossibly remember all the information ZocDoc provides. This is the big advantage of using their service plus of course that it's free and convenient to use.

I think two types of places should be considered for big billboards and brochures with additional info: doctor's offices and places with lots of travellers like train stations, airports and the subway.

The camapign could become more efficient with a person or mascot to represent ZocDoc as healthcare is something intimate and it helps if you can relate to the service in a more personal way. Most important this should be someone looking very trustworthy, wise and competent like, e.g. like a grandfather.