Jonah Goldberg has an interesting story on the "war on best friends."
As the father of two, I've seen my boys keep some friends, drop others over time and pick up new ones in new environments. Both would say they have a best friend, along with an orbit of good friends.
Sometimes, it's messy. When the youngest really feels he needs a friend to play with. As the oldest turns 13, he's content with his games. But enjoys being with the new friends he's made at middle school.
Hopefully, they are building friendships where they'll be keeping up on Facebook (or it's future form) in 30 years.