We have had photo contests via Instagram as well as a feed of all Freezing Saddles posts. If I’m not mistaken, I believe we hashtagged #bafs2015 (now would be 2016). I also used #freezingsaddles on my posts.
In asking what the hashtag is, you assume rather more organization than what we’ve yet managed. But I’m with kwarkentien; last year’s was #BAFS2015, so it would make sense to make this year’s #BAFS2016.