Facebook posted today about the new hashtag support with an overview, saying:
Starting today, hashtags will be clickable on Facebook. Similar to other services like Instagram, Twitter, Tumblr, or Pinterest, hashtags on Facebook allow you to add context to a post or indicate that it is part of a larger discussion. When you click on a hashtag in Facebook, you’ll see a feed of what other people and Pages are saying about that event or topic.
When you click on a hashtag, a new overlay window appears showing you matching content.
Ironically, while you still can’t search for posts in Facebook Graph Search, you can search for hashtags. Begin your search with any hashtag word, a word starting with the # symbol