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HTTPS encrypts content as it's transferred over a network, providing protection against attempts to access that data. Even if the network connection is insecure (like a public Wi-Fi hotspot), the traffic over the HTTPS connection is still encrypted and secure.
Foursquare announced the change over Twitter. Foursquare told us that it is moving to HTTPS to "proactively increase the security of all foursquare accounts."
Foursquare's move to HTTPS follows similar moves by Facebook and Twitter. Unlike those two services, however, Foursquare's HTTPS support is turned on by default. The log-in data associated with Foursquare might not be as important as say, Facebook, but the location information associated with the service is a different story.