More than 10 million people have signed up to Twitter’s livestreaming app, Periscope, since it was launched at the end of March, with the aim of letting everyone send livestreamed videos from their phone to the world.And now users are sending 40 years of video per day, according to Twitter, which owns Periscope.Periscope said the amount of time watched “is the metric we care about most”, because it tells it how successful it is for users and indicates how many people are using the app.