Foursquare started by asking users to check in, signal, and share their location. Groupon started by asking users to buy things online at steep discounts.
Foursquare eventually added features like deals/promotions, the ability to snap & geotag pictures, share tips, and struck a deal with AMEX. Groupon is rumored to be launching a mobile experience that may do the same, and more.
Foursquare hinted today that the app is going to start making recommendations based on all the places data it has indexed. Groupon also needs to personalize its deals beyond the location parameter.
Foursquare users began using the service because it was cool, there no promise of any immediate economic benefit or social status. Groupon users began using the site because there were immediate financial benefits to be had.
In other words, Foursquare users started on the site motivated by adventure. By contrast, Groupon users were motivated by savings.
These are two very different audiences with some interesting overlaps, and I am starting to feel that they are complementary in nature and, if joined, could be a potent match and threaten many existing commerce sites and services.