C.L.
I have two boys and they've had different experiences. My 9th grader kept pretty much his same group of friends for the first two years of middle school and ate lunch every day with boys from his elementary school. In 8th grade he started hanging out a little less with the neighborhood/elementary school boys and more with his sports and band friends. Now that he's in HS the trend towards spending more time with the band/sports friends is increasing.
My 6th grader has only one good friend he's hung onto from elementary school and has several new friends from middle school. He does have two really close friends he's met outside of school that he has maintained.
They both went to the same middle school which is very large and combines parts or all of 5 different elementary schools. I think as kids get older they spend more time and become closer to the kids who have their same interests and activities. That's what happened to me. My group of friends changed completely between elementary school and middle school/high school.