I'm going to say something potentially controversial here and claim that friendships are dynamic.
Some will last, some will not, for a variety of reasons. All you can really do is value the friendships that you have right now, and maybe try to make new ones.
I try not to put my friends into categories, even though I trust some of them more than others. There are a couple friends I would consider close friends.. but I consider them all true friends, to an extent. As soon as someone isn't a "true" friend, I call them an acquaintance.
So I suppose what I'm saying is only true because I fuddled with the definitions. (but I think I did so for a good reason)