The primary mode of revenue generation is business partnerships.
To understand how this works, let’s take an example. Suppose you went to Travel agent and he offers all sorts of travel packages and he doesn’t charge you! Now you might be wondering how does he makes money! What he does is that he contacts all the Transport and Hospitality service providers and asks them to give him a percentage of business he brings to them. So whenever you go to the travel agent and book tickets he will be generating revenue without you ever knowing.
In case of Foursquare, it’s with daily deal partnerships which local businesses from companies, foursquare offers some deal to it’s customers and charges from the company. And one more mode of revenue stream is marketing program partnerships, where it allows you to find special deals with specific payment cards/options.
Now you know why and how, a big consumer base matters while monetzing your product.
Whenever you see something like this on foursquare, you know that they just made some money!