I would do as the above mentioned and talk to her about what is going on. Not in a "there is something wrong with you because you are not wanting to hang out at parties and neighborhood functions" kind of way but a "hey, how are you feeling these days" kind of way.
She may be self conscious about something. Someone may have hurt her feelings. She may be developing low self esteem. She may be the kind of person that doesn't need or want a lot of people around and is just really acting on it now. I can be like this at times. I prefer a one on one vs. a party myself. Always have been that way.
I think that I'd not try to force her to attend these events right now. That could compound the situation. Instead, if she is still okay with one on one's-------- set them up. Set them up a few times a week but at least twice. I'd limit things like computer time which can isolate someone. And when she has the one on one time with her friend . . . be near by. Listen in. See what you think is going on and keep a dialogue open with her about it. I mean, don't let her know that you are watching her but just help her along if she is having some issues in her social skills or with friends in general.
I'd think of the fall and what she can be active in. Girl Scouts? Soccer? Art or music lessons? Dance or gymnastics? Get her in something once a quarter to help promote friendships. Even karate is a cool activity for a girl.
Good luck. But she may just be more of a person that likes alone time and that is okay. If you feel like she is depressed, start monitering it and take her to a psychiatrist that specializes in kids.
Well when I was eleven i was bullied alot at school and never told my mom about it and i stopped wanting to play with my friends but eventually i opened up and asked for new friends and i started to feel better. She could be a very shy and might have things in her mind locked away. Talk to her and see if she comes around.