Starting today, Facebook is officially adding support for the hashtag, allowing users to better follow and take part in conversation about specific topics.
Users can do this by searching or clicking on the #hashtags, opening a stream of updates tagged with the phrase. Results will be limited by privacy settings, allowing only public posts or content posted by your friends to be visible. Post with a hashtag and, much like Twitter, your post will be indicated as part of the wider discussion about that subject.
While this is a small change, it could make a big impact in terms of making Facebook more of a "live" social network, something which it has struggled with compared to other services.
Source: Facebook Newsroom