We love that Marc Jacobs CEO Robert Duffy is on Twitter, his tweets are refreshingly honest and insightful. Now the iconic fashion brand has hit up another social networking site – FourSquare, a site which allows you to share hotspots you’ve visited. And to make it worth your while to join the FourSquare Marc Jacobs fan page, any fan who visits any Marc by Marc Jacobs store in America will qualify for a gift with any purchase of $10 or more. For those who aren’t in America by have access to a Marc Jacobs boutique, you’ll also learn about the store’s neighborhood or a interesting fact about the company. Nifty. Now off we go in search of a FourSquare app for our Blackberry…