I will suggest that you investigate a couple of issues, both relating to water.

Are you watering sufficiently on watering day or limiting the amount that you pour in? Are you watering in such a manner that creates dry pockets throughout the soil volume?

Is your plant sitting in a drainage saucer filled with water for days?

Wow, that one is rough. Absooutely a watering issue and lack of light.

The leaves that are currently on it already look dead.

If any of the tree is still alive, you might be able to get it to come back if you put it outside for the summer and cut back all of the dead parts.

bottom line, too little or too much water cause the same affect: wilting and and leaf abscission. Both mean the plant can't take up water because the soil is too dry, or it's constantly too wet causing the roots to rot and no longer supply the canopy with water. Since you say the soil is moist all the time, that leads me to believe you're over watering. The soil needs to dry out between waterings.

depotting and inspecting the roots will tell you. If there are anything that are brown, soft to the tough and smell funny, that's rot.

If it doesn't make it and you'd like to try another one, I'd strongly suggest you do a lot of reading for before buying another. These two links will help.

http://forums2.gardenweb.com/discussions/1493078/ficus-trees-in-containers

http://forums2.gardenweb.com/discussions/2842847/container-soils-water-movement-and-retention-xxii?n=488