Zoopla's half year results filed today show the number of agents on its platform fell by 23% in the six months to March 31, equivalent to just under 4,000 people.
That's a lot of lost revenue for Zoopla, which charges agents a subscription of around £300 ($464) per month to list as many properties on the site as they want.
The big problem is the creation of OnTheMarket, a rival online property website launched in January by estate agent themselves. Agents are unhappy with the dominance of Zoopla and rival Rightmove and have launched a rival site that charges less to advertise and gives agents more say.
OnTheMarket is backed by Agents' Mutual, a not-for-profit backed by top estate agents including Savills, Knight Frank, Chestertons and Strutt & Parker.
One of OnTheMarket's policies is that anyone who lists property on the site can only feature it on one other property portal. Zoopla blamed this policy for hitting agent numbers, as subscribers to multiple site whittle down their numbers to join OnTheMarket.
Zoopla thinks it's only a temporary problem though. The company said 'churn levels' - the number of subscribers leaving the site over the period - "have slowed significantly over the past few months and are returning towards normal historic levels."
Zoopla is also managing to extract more cash from the agents that are staying, with average revenue per advertiser rising by 13% in the period. Overseas agents and commercial agents are also coming on to the platform.
These factors helped Zoopla grow revenue by 10% compared to last year to £42 million ($65.07 million). Earnings before exceptional costs rose by 14% to £21.4 million ($33.15 million).