The article mentions it, but the achievement aspect of Foursquare is a Very Significant Differentiator as opposed to a footnote.

You have two competing services with similar functionality and one is fun to use. Investors are betting that the fun to use one is the one that will catch on and stick now and in the future. The fact that the not-so-fun competitor is making money should make Foursquare investors jump for joy. It's a proof of business concept that's been done for them.