I live in Fort Worth Texas, arguably one of the largest and busiest cities in America, and I absolutely struggle with enjoying Streetpass and Local Multiplayer features. I can be away from home often for 8-10 hours and still come home with no street passes, let alone a streetpass for the actual game I wanted. Since Streetpass works in sleepmode, you can never tell if you're around fellow 3DS users, and the ONLY people I physically see with a 3DS in their hands are 7 year olds with Nintendogs, and what luck, I didn't brink my 3DS into work that day, so no streetpass for me. If you're like me and buy games on Day 1, you're often among the 1st a fewest that have that game, and you get bored of it and stop playing before it gets the chance to reach other players, not that it matters because as I said before they're 7.
The only time I got streetpasses was when I worked at Walmart and brought my 3DS everyday. I would average 0 to 3 streetpasses a day, most days being 0.