It's just part of life, out-growing your crowd and taking a different path in life - the ones that are true friends will always be around if you keep in touch. Over time people change and move on, settle down etc - just concentrate on what you are into work wise and what you want, don't give it up because it makes you unpopular! I remember when I left school, the gang split up gradually as people found work and grew up at different rates. The true friends are still there now, even if I don't see them that often.