The idea of sublingual use of a drug (letting it dissolve under your tongue) is that some drugs can be absorbed much more quickly by doing that. Perhaps the best known example of this is nitroglycerin tablets for heart problems; by placing the tablet under the tongue, you can get the drug into your system very quickly (which is important when you're having heart problems.)
BUT! Nitroglycerin is unusual in its ability to be absorbed quickly through the skin of your mouth. Most drugs can't be effectively absorbed this way, and I would bet that MDMA is one of them. What really happens is just that the pill gets dissolved by saliva and washed down into your stomach, so you do end up with the drug in your system, but it's not actually any different from just swallowing it. As others have said, holding a pill under your tongue like that can also be extremely irritating to the skin of your mouth, causing sores.
I would suggest just swallowing your pills. Sublingual is a waste of effort.