http://www.nytimes.com/2012/07/15/fashion/the-challenge-of-making-friends-as-an-adult.html?pagewanted=all

This is kind of old but I thought this was interesting. It talks about how easy it was to make life long friends in college mostly due to being in the same place in life pretty much but as an adult it's harder because everyone meets milestones at different points but not only that as parents we eventually want to spend more time with our kids than friends.

As someone that still has mostly high school and college friends (moved back to my home town after college). This made me feel better that I might not be the only having trouble finding new adult friends.