On the March 5, 2016 edition of 99% Invisible:
If you want to follow conversation threads relating to this show on social media—whether Twitter, Facebook, or Instagram, Tumblr—you know to look for the hashtag: #99pi. In our current digital age, the hashtag identifies movements, events, happenings, brands—topics of all kinds. The “#” didn’t always have this meaning, though. It’s had a few different lives.