For introverts social contact is simply EXHAUSTING. So there is a high cost to every social interaction. Hence why they avoid parties, or responding to your messages or any of the other things that extorverts seem to think are so "fun". I wish extroverts would understand this...
Doing things that you aren’t good at, that you haven’t effectively practiced, is tiring. If you aren’t an experienced driver, for instance, a lengthy, unfamiliar journey will feel exhausting. Once you get good, it’s second nature, and you don’t bat an eyelid.
It’s pretty simple. You don’t need to invent this concept about special types of people who are “tired” by social contact.
So introverts don't really get tired because they lack social skills and are trying -- but because that it takes alot of mental effort to continuously integrate all this new information.
If what you were saying is true then introverts would be less introverted over time as they practiced social skills but the opposite is often true.
Also if people tell you they are "tired" by social contact -- there isn't much you can do about it. Whether you believe it or not makes absolutely no difference to introverts. :)