We have had purple martins for years and years. It was difficult to get them started in MS and then again in TN but there are "clouds" of them at this moment because the babies are all hatching. Our vet here in GA has lots of them.
Ours usually do not leave the nest just because the babies can fly. They all leave late in the year and come back in the spring. A predator or something may have gotten yours.
We clean our houses in winter and don't put them up in the spring until we see the first junior scouts. It is always a fight then to keep the houses ready and the sparrows or black birds out. Once they have adopted your housing, they will multiply and be back every year. I love to listen to them "chitter" and watch them swope and glide all over.
There is much information online if you would like to learn about these beautiful birds.
http://purplemartins.com is a good place to start.