Probably you are aware about the importance of Facebook as the largest social network worldwide. Being an open network it correlates with being a general news source and being an important traffic source. This of course implies the importance of ranking factors. Some generally rather rare information about this issue is supplied by the articles below.
An article by Brian Solis refers to some resources with information about success factors on sharing news within the Facebook network, which may be described by optimization of copywriting. Beside simplicity some other extraordinary factors are listed like mentioning numbers within a headline.
The second article refers to information about the ranking factors Facebook uses to rank articles within its feed system. The mechanism is called EdgeRank and has been made public at the Facebook Developer Conference officially.
Additionally a ranking service displaying the most shared items on Facebook is provided by ItsTrending.com. The service is giving an overview by topics and media.