That's so baffling. A while back I used...I think it was Timbuk2's backpack/messenger bag customization interface, which is fantastic for actually designing/ordering a product and has a "share" button, but it inexplicably only lets you share to Facebook. (And maybe Twitter and G+, I forget.) They actually generate a URL, don't let you see it, but will put it on Facebook for you.

I cannot comprehend what sort of thought leads to an implementation like that.