Most of my friends are extroverts, but I'm an introvert
Those terms were popularized by Carl Jung,
How long ago?
Not sure, but they are typically viewed on a single contiuum.
Is there typically a 50/50 split between the two?
No it is widely varied. Extraverts tend to be outgoing and talkative, whereas introverts are more solitary and reserved.
How many extraverts are there compared to introverts?
Not sure, but everybody has both an introverted and extroverted side, its just one is more dominant.
I didn't realize that! Are there any other similar theories about it?
It is a central dimension of human personality theories.