Last night I was on Yelp checking out reviews of Spike Mendelsohn’ new Capital Hill pizza joint We, The Pizza (not good).  I have a Yelp account but wasn’t logged in, so noticed a clever way they are using Facebook’s API.  If you are not logged in to Yelp, you’ll see a note at the bottom of the page telling you how many of your Facebook friends are on Yelp and encouraging you to sign up. 

This is really smart.  Yelp is essentially a social network, so people are more likely to join and be active if their friends are already using the service.  Well done Yelp.