I think a lot of people are closer to the introverted side of things, but just show their extroverted side in public. Most people can't wait to get home, kick off their shoes and just relax imo.
If your second function is E, then you can still appear to be an extrovert even though your leading function is I,
I think you can be both, depending on which function you are using.
No I don't think there are. I think truly friendly and outgoing people are rare. I also think the answer is somewhere in between. And each individual has times of introversion and extroversion.