It may not surprise you to learn the following: people are more likely become friends with people they bump into often. Social psychology calls this propinquity, and it goes like this: if you're Bob's next door neighbor, and you pass each other all the time going in and out of the place, odds are you and Bob will become friends. If you and Bob have different schedules, it's less likely you'll become friends. If you and Bob live on different floors, but you both live near a stairway, you'll more likely become friends. You're more likely to stay friends if you're around each other more. Anyone who's moved has experienced the downside of this - that even if you're pretty close friends with someone, you may well fall out of touch if you don't easily run into each other often.
(A personal aside, for a moment: Of course, this has happened to me. I've just got off of Facebook (if you don't know what that is, type it in Google search or get off the Internet before you hurt yourself), after messaging a couple of relatively close friends from UC Berkeley, friends whom I have not spoken to in months. I graduated mere months ago; and already I've fallen out of touch with most of my friends there. Even those I was closest to - for the time being at least, I talk to them only sporadically.
Ah, guilt. It's so much fun.)
In fiction land, I rarely see this concept at work. Characters are more likely to become fast friends, and remain so barring a dramatic falling out) after bumping into each other once. This sounds perfectly logical - why wouldn't you want to be friends with someone you obviously get along with well? - but in Real Life, you may never really connect with that awesome person you met on the bus that one time, because even though you've got their phone number, you'll never run into them in your daily life.
It's an open question whether this is really a problem. For one fiction tends to depict the interesting rather than the everyday; 'propinquity' is firmly of the everyday. Becoming longtime friends with someone you hardly see happens in real life all the time; it's just that more of your friends than not will probably be people you run into all the time.
However, if/when this doesn't hold true for characters in fiction, I'm not sure anyone would notice. I probably wouldn't have even noticed this trope operating if I didn't watch so many sci-fi/fantasy ensemble shows, genres that aren't known for their psychological realism. In the TV series Angel, for instance - a series I love - sometimes you get the idea that Los Angeles is empty of all people except the main cast, demons/vampires/monsters, and victims. That goes for its parent show Buffy, too, depending on the season. These two shows are giants of the genre in the televised field.
So, how much should a fiction writer take propinquity into account? Is this the sort of thing that can safely - even happily - be ignored?