They post jobs all the time but never respond to resumes and they will keep the same job posted for weeks if not months. Are these actual jobs or just fake ads to get resumes in?
They can be pretty hit and miss but yes, they often are trying to fill real positions.
From experience, and previous employment, like all help wanted ads some are legit and others are total time wasters. Head hunters need to justify their time and billable hours, like all HR people, don't forget.