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 DorseyYou don't have to start from scratch to do something interesting. You don't have to start from scratch to have a massive impact on the world. You have to have a good idea. You have to convince other people of those good ideas. And you have to push as quickly as possible.
Jack Dorsey