I think it's easier to stay friends when you have shared experiences and have good memories which help with forming the bond, especially post school days.
Also, as per previous comments, making the effort is something we all need to do, even if it's just a pint or two at the pub!
Family don't really compare in my eyes, the dynamic is totally different (not necessarily better) as you can choose your friends but you're stuck with your family.
I think it's great to have friends, especially those from very different walks of life as you get a real appreciation of how different people's lives are. I've seen groups of friends who are basically carbon copies of each other and I find it a bit sad but that's their choice, so...