When taking my 3DS with me to and from work. 1 of which is during rush hour, in public transit.
By that, I would have streetpassed a lot of people if the 3DS were actually popular.
I also hardly ever see people playing on a 3DS other than small children.
Months ago, I would streetpass at least 1 person per day to/from work.
So, I could only conclude, base on my personal experience, that, the 3DS is doomed to gather dust at home.
Streetpass features failed hard on getting people to carry their 3DS on the go. People would rather play shooting birds on their iPad Nanos than on a 3DS.
To be fair, I also don't see anyone carrying a Vita. So, it's as much doomed as the 3DS is around here.