I can see where your coming from, it was much easier to make friends in school because you had more time to bond with them. But in school I didn't have many friends. Looking back that was most likely my fault, In school everyone else was caucassian and one korean person

(guess who) so I had to deal with quite a lot of teasing and it was hard for me to open up to people or even trust them.
I think we are kind of stuck with the friends we have after a certain age. It becomes harder to open up to people because we don't have the time. But it's not impossible to make them. I have had the same friends for years and I don't know if I could make new ones easily. No one can say who is more important it's your feelings about the people. The important thing is not to take them either of them for granted.
Now on the subject of acquaintances, I whole heartedly agree with Ian I hate acquaintances, if only for the fact when I am in a store I have to make false pleasantries with 2 or 3 people asking how they've been when I really don't care and asking what they are here for when I know what they are here for, they need food. Then making a useless comment on the weather. People shouldn't feel like they need to stop and say hello. I don't know how many times I have heard people say "Well I should just go say hello" they are just furthering the concept that everyone everywhere has to say hello.