I wasn't a very social teen. I'm still not a social adult In my case it was definitely because I felt left out and uncool and therefore out of self-preservation I preferred my own company to trying so hard to fit in and always being disappointed and hurt.
But you know what hurt more? I had to constantly hear about how my mom was super social at my age and had a million boyfriends and was always partying and blah blah. She didn't do it out of malice, more like encouragement. But one time when she thought I was napping in the family room I overheard her talking to her friend in the kitchen about how I had no friends. Overhearing that has stuck with me to this day; feeling like I wasn't good enough for my mom's standards of popularity really hurt.
In other words, let her be and find her own way. Continue to make her feel loved and wanted at home and she will be fine.