StreetPass is a nice little bit social networking from Nintendo.
You just pass by any random person, exchange some data, play games with them, etc.
However, when you turn on your 3DS for the first time, and go to StreetPass Mii Plaza, you'll be welcomed with the following questions:

Do you want to turn on StreetPass?
Of course we want, so say "Yes".

Now just choose the Mii you want to use.
If you don't have any, make one in Mii Maker/Studio.

Is this Mii okay?
Just say "It's good".
Seriously, who doesn't like his- or herself?

Share it with Mii Maker/Studio?
This one is up to you, but I have chosen "Yes".

Now you set up your welcome message.
Note: This one is disabled by default, so enable it yourself, afterwards.

Is this good enough?
Sure.....!

Forever alone? Then now just StreetPass!
Don't worry, just close StreetPass Mii Plaza (it must not be active, not even paused!), put your 3DS into Sleep Mode (close the lid of the 3DS), put it into your pocket, and let's take a walk.

4 hours later...
Now you've met a few 3DS's, congratulations!

 

[Thesolcity]

I can see it now...

ADS VIA STREETPASS

;O;

 

[YamiHoshi.nl]

Many people do that.
I've even got Advertised by an employee of the local Media Markt, through StreetPass.

 

[kirbymaster101]

May I ask what recording device you use? It has GODLY QUALITY!

 

[YamiHoshi.nl]

3DS Capture Board, it's currently in development, and I'm one of the Beta Testers of it.

 

[kirbymaster101]

well its awesome but I guess it involves soldering?

 

[YamiHoshi.nl]

Yes.