It’s a strange tradition, but like so many others Foursquare dropped their primary function, checking in to a place, from the app in order to concentrate on being something like Yelp. If you want to check in like you used to, you have to install a new app they call “Swarm”.
I declined, and since I already have multiple apps for recommending places I uninstalled Foursquare. Obviously, this is the opposite of the behavior they were looking for but as the linked article correctly points out, checking in was the entire point behind Foursquare. Conveniently, Facebook has a check in feature and since most of my friends are on Facebook, it’s more than adequate for my purposes.
If there’s a message in all of this it’s this: Don’t forget what made you popular to begin with. If you don’t know, ask. Then stick with it.