For now the best way to follow the tag is to either search for or click on an existing post with the tag, which will bring you to the Hashtag Page
According to the FB blog post announcing the features, the current rollout of the FB hashtag will allow:
Introducing Hashtags on Facebook
Starting today, hashtags will be clickable on Facebook... 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.
There will be a hashtag feed that you can currently search and/or click on to, but the follow feature itself hasn't fully been rolled out according to Facebook Gives Marketing Advice For New Hashtags
Facebook users will be able to search for a specific hashtag from the search bar and compose posts directly from the hashtag feed.
The Facebook hashtag system will be slightly different from Twitter since it will still be subject to the same privacy restrictions. From TechCrunch's Facebook Announces Searchable Hashtags, Promises More Features For Following Public Conversations
A company spokesperson told me that from a privacy perspective, hashtags will work similarly to Facebook Graph Search. In other words, you’ll only see the comments that you’re authorized to see. So if I include a hashtag in a friends-only post, then my friends will still be the only ones who can see it in a hashtag search.