My ficus was happiest when it was directly next to a northeast facing completely uncovered window, evenly moist soil and NO DRAFTS OF ANY KIND. Too bad I moved and can't provide those same conditions anymore but I've experienced most severe leaf drop when it gets too dry and/or is in any kind of breezy situation.

Ficuses are very finnicky about being moved. Every time you move one, even in your house, it will respond by dropping a bunch of leaves.

For best results, find a spot with bright, indirect light, away fron any heat vents or other drafts and leave it there! You can rotate it a 1/4 turn maybe once a month for symmertical growth. Misting also helps!
Look out for scale, spider mites, or other sucking insects. Keep it clean, (Yes, you can wash off the leaves with a very weak soap solution), don't overwater, and don't hurry to transplant it. A Ficus Benjamina tree is a member of the Rubber Plant family. Their roots will be all over, even on top of the soil. This does not necessarily mean the plant is rootbound! You could repot it, maybe, every 3 years....and.... watch the leaves drop again!!!

Dropping leaves on a ficus does not mean it is dying. If you leave it alone, many of the leaves will re-grow.
Newly purchased plants usually have such a lush growth of leaves. The plant cannot sustain them outside of a nursery, and an initial leaf-drop is to be expected. Cheer up!!! gitagal