The move was expected, as Foursquare had been using an ad sales pitch deck since April that touted check-in targeting and pixel tracking. (Pixel tracking is one way that users are followed on the web by advertisers.)
What's interesting about the choice of Turn is that the company is best-known as on of the adtech world's largest "demand-side platforms" (DSPs). A DSP is basically an ad-buying system, in which clients buy inventory in a real-time bidding auction. The inventory is usually triggered by tracking cookies, so buyers can target people by their apparent interests as indicated by their web surfing history.
Turn is a major player inside FBX,
At Foursquare, a users' interest will, obviously, be inferred from their check-in. The innovation here — if the implication in the ad sales pitch deck is correct — is that the ad will be triggered by the check-in. Turn can apply its database of cookies to match that check-in with a relevant audience.
It looks like Foursquare will not be offering individual users as targets, but rather anonymous blocks of users with similar interests, such as "mass market mom," "business traveler," and "luxury affinity."
See Foursquare's pitch deck here.
