Perhaps because at this time, Facebook just has more users, and not because Facebook has the better strategy for information dissemination, here is another independent source citing that Facebook referrals have been more numerous than Twitter referrals. In this case, it's a college newspaper, dealing with traffic spikes as a result of a breaking story.
Twitter can be an easy way to notify readers, but by far the best social networking site for you to focus on is Facebook. If there is a Facebook page or group concerning the news, post a link to your Web site. Have your reporters post links on their walls and Twitter accounts. Together, these two mediums can drive hundreds or thousands of visitors to your Web site. For example, the day news broke about the student suicide at Cal Poly, more than half of the Mustang Daily’s pageviews for the day were referred from Facebook (56.6 percent, to be exact).