Note this is not damage per say, serotonin is a neurotransmitter responsible for the regulation of mood (as well as many other things). What MDMA does is basically flood your brain with serotonin from the synapses which is then broken down by monoamine oxidase. If you were to continue repeatedly taking MDMA eventually your brain would run out of serotonin. The lack of this neurotransmitter will cause feelings of depression, this is not permenant however and your brain will eventually replenish itself.
Taking 5-HTP and supplements can help this process as it will be converted to serotonin in the body, I think niacin is too but don't take my word on that one.

The issue of actual MDMA neurotoxicity is still controversal. Although a man named George Ricaurte did research on primates which was supposed to prove it caused brain damage, him and his research team retracted their findings because they used methamphetamine, not MDMA in the experiments and claimed it to be a human error.

http://www.dancesafe.org/slideshow/ - This is your brain on ecstasy.