…when a Page tags a separate brand or celebrity Page in a post, that content may surface for followers of both Pages. For example, if Mashable posts a story to Facebook and tags Google’s Page, the post could now appear in News Feed for both Mashable fans and Google fans.
In short, it’s a simple way for brands or celebrities to greatly expand their audience.
This feature was already operational for updates from friends. If you tag someone in a photo, for example, their friends may see that photo in News Feed even if you are not connected to them.
The difference here is that while your photo could appear in front of hundreds of other people, a Page post that tags Google could appear in front of thousands, if not millions of additional users. (Google has more than 15 million users who have Liked its Page.)
Facebook says it received positive feedback after testing this feature with users. The algorithm will take into account which users are engaging with content to determine if it is relevant for fans of both the Page that posted it, and the Page that was tagged. The new feature only works between Pages, meaning the typical user could not expand his audience by tagging a Page when posting.
Kurt Wagner, Mashable – 2/25/2014