As a landlord, I try to respond to every inquiry. However, it's frustrating and a waste of everybody's time when Trulia doesn't promptly remove a listing after I notify them that the property has been rented. It seems that I spend more time responding with, "Sorry, already gone", than the time on inquires when the property is actually available. Trulia, Zillow and Hot Pads care about advertising, not listings. They try to keep listing posted as long as possible in order to generate more ad hits.
When the property is available, I get great applicants. Afterwards, it's a big waste of time.