I think a generally good way to think about it is whether someone is energized or drained by spending time with people. I am a social introvert: I like people, but it takes energy to be with people. When I am drained, I need to be alone. My brother is an extreme extrovert: he actually gains energy from being with people (which seems insane to me).
As an introvert I certainly can get lonely. Just might look different/happen in different circumstances to my extroverted brother.