A few weeks ago, I tweeted "Why not create a Facebook only news site". Rockville Central, a Maryland (US) small local news site, is doing it. I don't bet often but, for once, I'm ready to bet that it is not going to be the last one.
According to the Huffington Post, 45% of Rockville Central users have a Facebook account. In addition, the social network is the second traffic source for the local news site.
Great move and good luck.