In January, Facebook introduced Trending to help highlight the global conversations that happen on Facebook. With this new feature, it has added a new way for people to discover conversations that are taking place around topics that they are interested in.
Now, when a Page tags another Page, Facebook may show the post to some of the people who like or follow the tagged Page. For example, this post by the Bleacher Report might be shown in News Feed to people who follow or like Dwight Howard, in addition to people who follow or like the Bleacher Report.
The same this happens with updates from friends — if a friend tags you in a photo, your friends may see that photo in their News Feed even if they’re not friends with the person who tagged you. When we tested adding this feature for Pages we found that people liked seeing this type of content in their News Feeds and gave these stories high scores in surveys.
Facebook looks at many factors to make sure the most relevant stories appear in News Feed, including which posts are getting the most engagement (such as likes, comments, shares and clicks) across all of Facebook network. They also consider which posts are getting the most engagement from people who like both the Page that posted and the Page that was tagged.
For example, if many people who like Dwight Howard also like the Bleacher Report, it suggests that these two Pages are connected. If we see that people who like both the Bleacher Report and Dwight Howard are liking the post above, that’s an indication that it may be relevant for people who only like Dwight Howard.
This means some Page posts that tag other Pages may be seen by new people.(Source : http://newsroom.fb.com, By Andrew Song, Product Manager, Facebook)