At least Foursquare is mobile-native, which is one of the primary goals of Yahoo. One of the primary things Foursquare desperately needs is scale, which is one thing Yahoo can legitimately claim to have.
For Foursquare it's a bit like admitting defeat, but being eaten by Yahoo could be a good move for users. I've been a foursquare fan since 2011 and would love if the service could regain some of its former luster and glory!