I rated myself as a solid introvert until I realized that I can be rather outgoing, but that sometimes my energy drains more easily than usual and it had little to do with E/I. After paying attention I realized my energy goes down the drain if I am around extroverts who are
Feelers. Otherwise, toss me with an EXTX (who aren't emotionally charged) and we can talk just fine, the energy I expend is just like I when I'm talking to introverts.
It also depends on what you like talking about, I really like sharing information (of almost any kind) so be it typing paragraphs of typology theories in this forum
(look at this self-fulfilling prophecy) or drafting a game strategy / guide, I can be rather talkative.
Most people are rather balanced. Think about it, most people would get along with people of similar interests, and avoid those who don't. Those who absolutely avoid people or befriend literally every one they see might be special snowflakes. Also, I vs E goes beyond sociability, it also has to do with energy / activity level in general, how much stimulation you need.
MBTI(R) Step II: The 40 Facets of the 16 Types – Personality Playbook
Most people aren't strictly either camp, but I find that this sheds some light into the division of the facets and what makes up Introversion and Extroversion.