If you open up Foursquare in your browser, you'll notice that it's no longer using the familiar Google Maps API. As of today, the company is switching to MapBox Streets, which builds maps based on data from crowdsourced atlas OpenStreetMap. After experimenting with creating its own maps with OSM in January, Foursquare decided to switch to MapBox's solution both because of the cost of using Google Maps and because of the flexibility it says the service offers. If you're using the Foursquare app on your phone, you won't notice any difference — those apps use whatever mapping tool is built into the OS, meaning iOS and Android users will still see Google Maps.
Since last year, Google has charged high-traffic commercial sites a fee for the Maps API regardless of whether or not they're a part of the paid premium service, something that has prompted a few services to look for an alternative. Some sites with a paid license have also opted to start building their own maps with free data to save money. A few major services, like WordPress and MapQuest, make use of OpenStreetMap data, but Foursquare is one of the most well-known sites so far to announce a switch.