I've read 'Personality Types' (the book where Jung define introversion and extraversion) many times by now. It is hard to understand at first, by now I am amazed about how his concepts are interpreted differently than what he intended. Introversion is a kind of filtering mechanism where your self concept (ego) overweight the importance of internal stimulis over external one (and vice versa for an extrovert).

If a subject feels 'obligated' to respond to an external stimulus (such as a party) and to expend energy in that direction, that is extraversion. If you need to learn how to monitor internal stimulus such as fatigue levels or who makes you feel good, that sound like an extravert learning how to do introversion in order to manage himself. This process of learning how to get out of your usual favored way is part of what Jung called 'individuation', psychological development which starts in adulthood.

When trying to understand the intro-extra spectrum, its not the behavior that matters the most, it is the cognition of the subject. Why did you stay alone last friday night? Is it because you had a hard week and you are tired (internal stimulus)? Do you avoid parties because you do not care that much about the feeling of the music (internal stimulus), and would rather to clearly hear others people stories and opinions (external stimulus)? How does the subject narrates his story?