My son is in Kindergarten, and has made a bunch of good friends. He has playdates with some of them sometimes, and one of us will go and get a chance to meet his friend's parents.

My parents knew the parents of my friends who lived on my street, and the ones I met in elementary school. By high school though, they had pretty much stopped keeping track of what I was up to! They knew most of my friends though, just because I made most of my HS friends when I was really young.