That is not a graph of the Foursquare app's popularity. It's a graph of Google searches. It was in trouble before it split the apps.
Lots of searches are driven by news stories, including bad news stories. For example the marker D is a story about how Foursquare resorted to raising debt financing to avoid a down round. It's a mistake to label that "SUCCESS".
BTW, the first time I followed your link, Flickr redirected me to an ad for a Flickr app for an operating system I don’t use. You might want to find new hosting for your images.
Lots of searches are driven by news stories, including bad news stories. For example the marker D is a story about how Foursquare resorted to raising debt financing to avoid a down round. It's a mistake to label that "SUCCESS".