Actually introverts have great advantages for learning a language. They have more analytical mind, they can consentrate better, they keep their goals better during the lesson, do not distract to chat in vain. Provided a focused practice they like talking about what’s important for the real progress, whereas extroverts like talking about everything else rather than what’s needed. They listen more carefully, whereas extaverts tend to say things faster, not caring if it’s correct. Besides, on Skype it feels different. You are in your room, in your chair, looking at your computer. If they find a good teacher, they are comfortable with, they are more motivated to learn long term. , but extraverts jump from one teacher to another.