I think it's more than a one-dimensional thing. Like the idea of the "political compass". Just as a person's political views cannot be compressed to a single "left - right axis", their social preferences cannot be reduced to a single "introvert - extrovert axis".

How is a person with existing relationships vs. new people? Similar people vs. different people?

My theory is that a lot of people labelled "introvert" are just more selective then average in who they want to deal with.