Friends and emotional attachments

How do we define people around us? Unknown, people we know by name, people we spend time with (but don't actually know), friends? Particularly the last two categories fascinate me. You get from category #1 to category #2 just by introducing and maybe sitting in the same class room. To number #3 by going for a beer together. But to #4?
Is there an universal definition? Some people say that once you let people into your home you are friends. Or that you can spend time together without actually doing anything. Are mutual interests expected? Trust? Love? Respect? Something else?

I'm asking this because while there are many people I happily declare my friends in all the above definitions of the word, there are some that I don't. Or, to put it less bluntly, don't think if it's proper to call them friends. I'll get back to this shortly. Bear with me.

I remember that at the age of 12 an adult asked me who my friends (ystävät) are. I responded by listing my few mates (kaverit). After that, the adult in question was careful to call them as such. The distinction was that "mate" is a #3 while "friend" is a #4. I notice that some people are more relaxed with these terms. I have often wondered do they do so just out of courtesy, or are they so quick to forge the emotional bonds that friends have between each other? Or imagine that such bonds exist? Or maybe they just have a definition that allows #3's to be called #4's.

I wonder.

To go on; is there a proper way to make a friend? I have never been very good at this. A person is an occasional distraction at school... and few years later you notice its four in the morning and you have spent forty-odd hours in the "distraction's" apartment watching television, telling poor jokes and talking about life. Obviously, at some part between these two scenes the relationship changed.

Back to the definition. I hear that people who use MySpace and MSN to forge social contacts make a distinction between "friend" and "friendster". By definition, the latter are not real friends, but #3's. It comes to me, can one forge real, lasting relationships through the Internet, without actually ever meeting the other face-to-face.
I remember reading last year in newspaper about a Finnish guy who had entered exchange of letters with American girl in the 1950s (when both of them were still in middle school) and only met the week the newspaper came out. They had written each other letters for over 50 years, diligently and without fail. And while the Finnish guy had been to America - indeed, only few score kilometres from the girl, he hadn't had the opportunity to see her. And she had travelled Europe but had likewise been unable to visit Finland.
Were they friends? I imagine they wrote on paper everything that happened in their lives. I imagine, that over the years they told each other secrets they didn't tell anyone else, not even their spouses. But they had never met. They didn't know how the other laughed, how the other smiled. Didn't know how the other liked to form sentences or what syllables they give weight when talking. Or how the other has curious way of bending head when checking the time.

Are such things important for friendship? Is it strange, pathetic or otherwise improper to call people you have talked daily for years friends? Even if you have never met, even if it's only through IRC or instant messenger?

Where do you draw the line? What's the definition?