For as long as I have been aware of putting people into categories there have been introverts and there have been extroverts. Some evaluations put me into the introvert class, and some put me into the extrovert class. I have never felt comfortable with either class. Now (as of this April) we have some internet articles about a new class of "ambiverts," and a new test to see if you are one (see below). Most of the questions on the test are subjective. Like most people, I like spending time in certain kinds of groups, but not others. Like most people, I enjoy spending time with my own thoughts when it is my choice to do so. There are times when I'm assertive, and times when I am not. It all depends on the circumstances.
According to this test, I come out squarely on the ambivert side (I prefer the term "omnivert") because all of the MBTI criteria for introverts and extroverts seem to apply to me at one time or another. I imagine that I'm not alone.
Given the choice of introvert, extrovert, and ambivert, it seems that most performing musicians are more likely to fit in the "ambivert" category. Perhaps it's because we seek out situations (some which are downright uncomfortable) where we can open ourselves up in front of strangers. Sometimes we draw energy from people, and sometimes we feel like they draw it from us.
The research that fed the above Forbes article comes from a dissertation by Wharton School of Business graduate student Adam M. Grant that is based on the productivity of salespeople working in call centers. Perhaps, given the nature of this study, we shouldn't take the new classification that much to heart. There must be hundreds or thousands of different personality "types" that would fit into the Ambi or Omni variety (leaders vs. followers would be two of them). Culture, family, physical make-up, affluence, class, geography, education, interests, habits, and understanding social cues play a great part in the way each one of us feels around others.