[thensome]

My mom and I are both like this. There's enough to do alone. I think there's also a lot of ambient cultural pressure (at least in the US) to socialize pretty frequently compared to other locations or past times. Obviously there were always people who were super extroverted and would be hosting parties and showing up at their friends' homes all the time, but back when there were only letters and maybe telegrams it would have taken a lot longer to plan to meet.

Something that I've actually enjoyed my whole life is writing letters. (and writing online)

Anyway, to answer the question more directly, I think it's common but those of us who do it all end up being considered abnormal by someone. :P what are you gonna do lol