Selling t-shirts for a site?

[Jeff]

If you are promoting the company Facebook itself, then yes, you probably would need permission. If you have a Facebook page, you can always put something on your shirt like, "Friend us on Facebook at: http://www.facebook.com/yourpage". If you use any icon images, be sure you either create it, own the rights to the image, or have a creative commons that gives permission to use it for what you have going. Or you could put a QR code image on your shirt saying, "Find me on Facebook! Scan this code with your smartphone!" or something like that.....

....Of course, then you might get creepy people coming up to you all day long with cell phones scanning your shirt. Just be careful where you put that QR code I guess!