It's a matter of invitations versus context. Twitter is really good at providing context, like, I''m having coffee at Third Rail Coffee.' Foursquare is about invitations to places. In this respect Foursquare has started to replace Yelp for me.
Jack DorseyThe interesting products out on the Internet today are not building new technologies. They're combining technologies. Instagram, for instance: Photos plus geolocation plus filters. Foursquare: restaurant reviews plus check-ins plus geo.
Jack Dorsey