The Wall Street Journal took this screenshot of the new Trending feature.
A Facebook spokesperson stated:
We are running a small test of a unit on News Feed that displays topics currently trending on Facebook. Right now it’s only available to a small percentage of US users and it is still in the early stages of development. We will share more details down the line if we decide to roll it out more widely.
This should be an interesting add-on for Facebook in their ever growing competition with Twitter.