Where are the purple martins?

I have had a purple martin house up for over 3 years now and I religiously clean out any nests from the sparrows. What more can I do, and how long does it take usually for them to find you?

It took me 5 years before I got any purple martins nesting in my birdhouse, but they have came back every year since. I was pretty dilegent in keeping the martin house cleaned out of any other birds nesting materials. Good luck, it is worth the wait.