Foursquare is a location based service where you "check-in".  It's a way of friends seeing where you are and perhaps be able to see other people you may know who are close to your location.  One of my good friends uses Foursquare every time we go out to lunch and it's something that I have never picked up.  I guess it's something I should do as I'm probably going to post or tweet about after the fact.

Pinterest is a photo sharing app that has never won me over.  I think it's a good service but it has never appealed to me.  

Instagram is perhaps my favorite social media platform to date.  I am an avid Instagram user and try to post every day.  I think it's a great way to connect with new people and to showcase art very well.  Models, musicians, photographers all use Instagram with a passion because it acts as a great portfolio for things you have done.