Firstly, I define a friend as someone who is not family who I am friends with, and we are in semi regular contact via phone, or meeting to hang out. If I am friendly with you but the only reason we even see each other is we work in the same place/go to church together, go to same gym, whatever then you are an acquaintance. As in the friendship does not extend to real meaningful contact outside of the common setting. If I changed jobs in other words I would most likely never contact you and vice versa.
So I have moved around a lot and hence have lost track of a lot of friends over the years. I do not keep up friendships well over distance (few do actually). I do work on building new circles of friendship. Had a few friends over the weekend for instance. Most of the people who came also have us over now and then (life is busy though). I do find that it is much easier once you are married with kids if you find parents of your kids friends who you have stuff in common with. Still, friendship is hard, especially if like me you have a large family who are relatively close as just keeping up with them somewhat fills the calendar. I am however good friends with all my direct family.