I was going to start a similar thread a few days ago, because there seem to be so many different definitions of friendship on here and I think the differences inform some of our disagreements about etiquette matters concerning friends.
To me, a friend is someone you spend quite a bit of time with, see regularly, and would do things for with no expectation of reciprocity or payment or or or. For example, I have a good friend, probably my closest up here, who is in hospital at the moment. I took her in there when she became ill. Kept her elderly mum informed. I've been going to see her most days to keep her company because I know she's miserable, doing her washing for her, taking her what she needs, etc. That to me is friendship, that's what friends do for each other. So, I am puzzled when I see things like (paraphrasing, obviously) "I wouldn't accommodate Friend X's dietary issues because it would ruin the look of my dinner party" or "All my bridesmaids must have the same hair cut because my wedding should be more important than how they prefer their hair to look". That to me is not friendship, because in my mind, a friend's feelings and/or inclusion is more important than appearances, and I am always surprised when this isn't the case for other people.
A lot of people in the posters' lives that I see described here as "friends" I would not term friends, but "acquaintances". People you see at the gym, at your local pub, whatever. I have a lot of these types of people in my life because of my involvement in the local amateur music scene - bands and orchestras etc. I'm friendly with them when I see them at rehearsals and we chat and have fun at the time and I'm Facebook friends with many of them, but that's different to being friends with them; there's no effort to see them at any other time other than when we'd be together for a rehearsal/gig/other band activity, so there's no deepening of the connection. Acquaintances aren't the level of friend that I would go out and do things with or have them over to my house for dinner, I just see them in the course of doing other things. Hence, many of these etiquette things never come up for me, because to me a friend's feelings are *always* going to be more important than, for example, having a dinner party that they can't be invited to for whatever reason, because that's what friendship means, in my mind.
My definition of friends has also changed as I've got older. When I was younger, my friends were very much as you describe, Bijou. Schoolfriends, people I went to the pub with, partied with - people who just happened to be 'there'. I'm still in touch with a lot of them as we've caught up via FB etc, but there's no deep connection because we simply don't have anything in common any more. I might see them when I go home - a group of us would perhaps catch up over a meal out perhaps once every couple of years, and it's always a pleasant way to spend an evening, but there's no desire to maintain that connection aside from at those times.
Nowadays, my circle is smaller, but closer, to the level described above, and I like it like that, because it feels meaningful.
Clear as mud, I expect, but I know what I'm trying to say!