I know that posting on forums has helped my shyness in person. I used to be extremely shy when I was younger, but you would never know it if you talked to me now. I can formulate my thoughts a lot better when I write them down rather than talking them out, and posting on forums helps me to form my opinions a lot more effectively before I have to discuss them with anyone in person (I am a big debater online). Also, I run a military wives forum (and I am a member of a few others) and have had the opportunity to help a good number of other military wives when they needed it, which has in turn made me feel good about myself. Not to mention that I have become a lot more confident in my own strengths as far as personality traits go, and that has definitely leaked into my life off the forums. I have made countless friends who love me for my personality instead of judging me for how I look (a big concern for girls a lot of times), and I have built self confidence because of it.
I don't think that forumming teaches you how to talk to people. The only thing that can teach you that is going out and doing it. The more you do it, the easier it gets. I do think that forumming can help you get in touch with who you are because all you have is your words, no clothes, make-up, or status symbols to hide behind.