Social development in childhood starts out with the parents. Most children are only social with their parents to start out with because that is who they are most comfortable with. This eventually branches out into other family member because this is who the child gets to see most often.
Adolescence is really where a person really starts to branch out even more socially. The kids start going to school and become friends with other kids. As they grow older into middle school, they start going to other kid's birthday parties and other events. Then comes the step that's right in between adolescence and adulthood, during the high school years where socially, I believe people are at the highest point, where kids in high school are doing more sporting events and becoming a little more independent and their social circles grow very large.
Early in adulthood, like in college or right after high school, people's social circles are still relatively large. As time goes on in adulthood, the social circles get smaller and smaller and pretty soon adults know who they are socially involved with mostly on a day-to-day or week-to-week basis.