You might have noticed that Facebook profiles now include a "Likes" box below the "Friends" box. It includes a count and list of which pages each user likes: Very straightforward. More interesting is that you can also view "mutual likes": the overlapping likes of that person and you.
That too is straightforward - but it represents something much bigger. Likes will (and I use that word intentionally / carefully) become criteria for how we browse and search the web. As Likes extend beyond Facebook (you can now like pages, articles, sites, etc), the habits of your social graph will determine your content experience (finding, reading, etc) ... all using your 'like graph'.
It is already happening on websites: for instance, if you visit the New York Times, it will suggest articles read by your Facebook peers. That assumes though that you visited NYT.com or saw the feed update on Facebook. Now imagine a search experience or engine predicated on cross-site likes.