I have 13 yo,who doesn't really have many friends,spends most of her time on the computer,though has excellent grades.She is avoiding people,crowds. I think in our case it is our family relationship,more precisely relationship between my husband and myself,as well as grandparental selfishness ,which is affecting her and stripping down whatever self esteem she has.I know she has not been abused ,she is not really bullied in the school.
I guess I'm trying to say,look into all that.Depression,no selfesteem,avoidance can be the cause of it. Sometimes kids prefer the computer because they can converse easier through PC,then in real life,which is tough on us -parents.
good luck