[toffer]

I found it surprising too, but your impression is wrong.

When you query the Foursquare API for a user, Foursquare returns the user's Facebook and Twitter names in their response (if the user has provided them to Foursquare):

https://developer.foursquare.com/docs/responses/user (see the "contact" field)

Once you have that, you just query Facebook and Twitter for their publicly available profile, photos, likes, tweets, etc.

[asmithmd1]

The Foursquare API explorer is the best demo of an API I have ever seen: https://developer.foursquare.com/docs/explore

[klausa]

That would explain it, thanks.

It seems it actually links to your facebook/twitter from your profile page, at least it does on webapp, I'm not sure how it is on mobile.

I think that's wrong default and should be changed.