speaking as a formerly shy child myself the worst thing you can do is pressure them or push them. It adds to their anxiety and makes the shyness worse. I did better as an adult with dealing with it because I was able to develop more understanding. Encourage social situations without pressuring your child and if your child is uncomfortable in overly social situations or is "shy" - it's perfectly fine. There are many of us who are shy, introverted types and we have friends and just the amount of social skills we need and feel comfortable with.
My parents, teachers etc. always pushed me to be more outgoing etc. and it added to the pressures I already felt and actually led me to be a "follower" rather than a leader in school because I felt I wasn't normal and had to do what others were doing for acceptance etc. This landed me in a lot of trouble. My son is also introverted and he does fine and has a strong sense of self because we don't pressure him about it. We realize it's just how some people are and you do outgrow it naturally if people don't make a big deal out of it.