s: "hey! I haven't seen you in ages! wow, crazy running into you like this; how have you been?! gosh, i would love to catch up, but i'm actually on my way to a meeting downtown. lemme get your phone number. i'll call you; we'll get some lunch."
a: "uh, okay. it's--"
and right at this point i wonder if i can get away with giving this guy the wrong phone number. but then what if he calls straight back so that i'll have his?
"--415.423.7798." gave out the real one.
s: "great, hold on real quick. let me call you back so you'll have my number too."
that would've been awkward. but now i got this dude's number, and, frankly, i don't even remember who he is. And so, here we arrive at another instance of a social retardation: what the hell is up with 'lunch'? nobody actually plans on meeting for these so-called 'lunches.' they don't even plan to call you. and, that's fine, really, because i don't plan on calling them either. so why bother going through the effort of looking like you care? it's not like saying hello and then moving on is rude. on the contrary, i find it sincere. i'd rather have a short exchange with someone i never see than a long one that gets pinned to the unreal expectation of meeting for a lunch that will never happen with a person i don't really know.
in fact, suggesting to do the 'lunch' thing makes you look like a schmuck. you stare someone in the face, act happy and surprised to see them, then even go so far as to suggest the two of you make an opportunity out of this chance encounter by meeting later to 'catch up'--and none of it's true. what's really being said is: "do i know you? i think i do. do i want to try and know you more? eh...not really. i deduced your current status via your appearance at present. you seem to be doing fine. there's really no need to draw this out any further just to see it go nowhere because we all know that's what's going to happen. we weren't close friends before; why start now?"
suggesting to do something with the intention of never doing it just for the sole purpose of coming off as nice: that's what's called being an asshole.
in the future, don't be surprised if this happens:
s: "let's do lunch."
a: "go fuck yourself."