Um, well, there's this?
I guess the gist of it is that, if you log in to a wifi hotspot, the 3DS will mark it on the Nintendo Street Pass server. I guess you'll get streetpass hits for some (or all?) of the people who have been there for some period, and people can get streetpass hits from you the same way.
Dunno how long that data is kept. Visit to visit, a few days, etc. If you frequent any places that have free wifi, it might get you hits.