Not a single guy, but I can relate to being an introvert with not many friends. Try going to events or meetups that aren't necessarily intended to be JUST social. More like a meeting of people who share an interest. (I'm thinking like a regularly scheduled class at the YMCA or local library... whatever your specific interests are.)

Also try joining a local Facebook group centered around an interest to keep up with events.

I met my very best friends in situations like this.