I have lost touch with almost everyone I knew at school and college. Most went off to University (I didn't) or I was just never good enough friends with them for either of us to make the effort. In the 80s the only ways to keep in touch were by telephone or letter, if they weren't local.
Those who went to Uni often met their future spouses there, or settled out of the area for other reasons, so didn't come back to their teenage stamping grounds very often. My circle of friends at school was never huge, but it sorted the wheat from the chaff when one had to put effort into keeping in touch when we no longer saw each other every day; and now I am in regular contact with only one person from college days (who is in fact now my best friend). Some have come back into contact but have not stayed as we have nothing in common now, other than having been friends at school.
Now teenagers have Facebook and other networking sites which take no account of distance, they can keep in touch easily and with minimum effort, with tens (and in some cases hundreds) of school friends. Will they stay 'friends' with them all because they never lose touch? or will friends naturally fall by the wayside in the same way? I imagine it will take longer than when one had to pick up a phone and pay for a call, or take the trouble to write and post a letter.