For me, friends come and go with shocking regularity. Then again, I have moved around frequently: once when I was 4 and barely able to remember my nursery friends; then again at age 12 when leaving to live in another city; once again when 18 to start University; and several times since leaving Uni and starting working - twice within Scotland, then when I moved back to England in 2004, and about once more since then, in 2007.
Each time I have held onto a few select friends, some of whom I lost contact with months afterwards, some of whom I run into again after years who remember me well (I bumped into one as recently as last week in fact) or whom I meet by chance.
Maybe it's because I've been a frequent mover that I've never felt any particular sense of loss when leaving my friends behind. I feel I know that some time in the future I will meet them again under different, maybe even bizarre circumstances, and I have managed to get used to those changes. Perhaps these bonds I've made have never been strong bonds, but strong enough to form a good and memorable friendship with many, and loose enough to not feel a great sense of loss when I finally do depart. Perhaps that's why I've never had a long-term friendship - or anything closer than that - with anyone before.
On the other hand, I have found it easier than I thought I would to meet new people and start making new friends, although sometimes it is hard to fit in with a new close-knit cohort easily, as I'm always the "friendly-faced outsider" in many groups.
I am with Kestra on this one: friends come and go and it's a case of making new friends and perhaps, with time, reforging the bonds with the old friends. I will admit that this point of view has been influenced by my travelling throughout life.
I am a little jealous of this. It takes me forever to make even one friend. People are at least for half a year in the "good acquaintance" category before they are called friends. It would be so much easier sometimes to make connections more quickly, although, like you already said, closeness might suffer from it.