Periscope was a brilliant streaming app. And one of the first that you could write messages to people who were streaming.

It was an incredibly intimate experience to write something, and see people in the video you're watching, react to it live. Seems normal now, with so many streaming apps, but periscope was the first I saw on mobile do this well.

Eventually, the big apps implemented it too (Facebook, Snapchat... Instagram). Which killed Periscope...