Generally, I just write a note I would like to share with a large number of people (maybe something on special stationery or the web address to my twitter or website) and then let people get it. Also, I sometimes include pictures/art or sound files they can download and share as well. I mean, since you can't control who gets it, why not just send something neat everyone can enjoy?
Also, this service is nothing like pictochat. I never understood the purpose of that (though they include a better version of it in Pokemon, so I can always just play it there if I want).
It can be fun as long as you do it in advance and set things up right.
Honestly, I've always found StreetPass annoying to be honest, it seems like it should work like email or instant messenger but doesn't, and that throws me off.
its not like instant messenger at all. All it is, is just posting notes. Think of it as a bulletin board for the 3DS. Streetpass never worked for me so I spotpass all my drawings. you gotta like to draw to get into it really.