The Premier Agent program is the primary driver of ad revenue for Zillow and was responsible for 71% of the top line in the first quarter. The company recently announced that it had enabled a new feature for its advertising platform. Last year, agents advertised on Zillow by ZIP code, paying a fixed fee determined by a ZIP code's popularity based on lead generation. This year, the process moved to an auction-based system where the advertising fee would be determined by the agents themselves. The higher the bid, the greater the share or number of ad impressions where the agent's contact info would appear.