obligate introvert
Someone who "opts out" of plenty potential social situations, regardless of whether they do want to participate or not—opts out not by their own will, but because they "have to" because of unchangeable or similar circumstances.
Susie is an obligate introvert; instead of following a proper public school education, due to a myriad of sensible reasons, she learns shit at home using Acellus, Wikipedia, her parents' biased political opinions, reality shows, and homeschool textbooks. The downside or "opportunity cost" of this is that she is unable to make friends face-to-face at school. She instead meets them mostly on the internet...