A couple weeks ago, I wrote about Facebook's mobile "Install" ad units now driving movie downloads. Over the Christmas holiday, another format arrived: "Shop Now". The unit itself is the same: a glossy, mid-stream ad that asks users to "Shop Now" (rather than "Install" or "Download"). Clicking the units keeps you inside the Facebook Application - which is a little confusing since it is an App Store icon and says "open in app". The landing page is a gift store for iTunes gift cards - like other Facebook digital gifts.
The interesting point is less around the unit / action itself... rather, it is that Facebook has successfully created single in-stream, mobile-only unit that is flexible enough to drive "Likes" (purchased by brands), drive Installs (purchased by developers), drive Downloads (purchased by media) and now drive sales (purchased by e-commerce companies).