Based on a sample of 17 highly-trafficked media outlets, Dan Zarella suggests that stories published in the morning have the greatest chance of being shared on Facebook.
Zarella tracked the stories posted to social media sites since February 2010, and determined that the peak sharing hour on Facebook was 9 a.m. EST. His sample includes articles from Huffington Post, The New York Times, CNN, Mashable, ESPN and others.
His report also indicates that while sharing is most common at 9 a.m., article publication actually peaks two hours later, at 11 a.m.
His results are not necessarily surprising as most news websites see a similar peak in overall traffic in the morning as people arrive at work. However, earlier research by Zarella also reported that re-tweeting activity (though not just of news) spikes around Noon.