Interesting post...
I've always seen friendship as having two different definitions or stages. True friendship, and Acquaintance. If anyone were to ask me 'How many friends do you have?' I'd say loads, because I do have lots of friends. However, only and handful of those are people I'd trust with my inner feelings/secrets/problems. Friendship in a way is an over-used and over-generalised word, because it can mean so many different things.
From what I've thought about, friendship is broken down into two important 'substances'. Mutual interest and Love.
Friendships generally start out with mutual interest, be it actual interest or a situation, such as school where you share the same experiences. If that interest or situation finishes/ends, people tend to drift apart and whatever it was that held the friendship together disappears. Unless you put love into the mix.
Sometimes, be it via extended mutual interest or by a few trying experiences, love develops and a bond is formed. I've known my best friend since we were both four years old, and even though we've grown up to be pretty much opposites with very little in common, the friendship has held purely because of love. She's always been there for me with my problems, and I've always been there for her.
At school I got on really well with everybody, be they the 'popular' people or the 'not so popular people'. However, I talk to very few of them now, in fact I think I'm only still in contact with two of them, my best friend and someone I happen to work with XD
The rest of those 'friendships' went no deeper than exchanged mutual interest, so when it was taken out of the equation, there was nothing else there.
So that's my theory, occasionally love develops, be it through experience, compatibility or circumstance. Those are the friends that stay with you throughout life. There's only one friend from work that I can honestly say will be my friend if either of us change jobs. While I get on with the rest of my co-workers, and call them friends, without work we'd have very little in common.
This goes on a lot. I remember at school watching some kids trying to be friends with the most popular ones, and pretty much just ending up being their bag-carriers. They seemed to deem it friendship, while the popular kids laughed at them and thought they were stupid. While I don't know what that feels like, since I was pretty much friendly with everyone, I chose to spend my school time with those that shared a couple of my interests, who may not have been very popular, but certainly made my school life tolerable.
In conclusion, before this post gets too long, I'd say that those are my views. I have a few true friends and many, many acquaintances. I think both are important, acquaintances to make life that much easier, and true friends to pick you up when you've fallen along the way. Me and my best friend benefit equally from our relationship. She's my rock... and my only true female company, as all my other real life 'true friends' are guys XD