Generally just doing things like volunteering or going places isn't enough. You have to be confident and assertive and talk to people, it's just the way it is. The main reason extroverts tend to have more friends is because they enjoy socializing and just talk to randoms. I only have one good friend, but I'm sure I could make more if I actually started striking up conversations with people at work/university.

The main places people get their friends are from their workplace, so just start a conversation with someone and talk about things that interest both of you. Most people who miss out on friends, such as myself, do so because they don't talk to anyone, even when they're in a social setting, and thus they leave not having made any new friends. So just put yourself out there and start talking to people at work/school/wherever.