A new video investigation by the Wall Street Journal finds the key to TikTok’s success in how the short-video sharing app monitors viewing times. From a report: TikTok is known for the fiendishly effective way that it selects streams of videos tailored to each user’s taste. The algorithm behind this personalization is the company’s prize asset — and, like those that power Google and Facebook, it’s a secret. WSJ created a batch of individualized dummy accounts to throw at TikTok and test how it homed in on each fake persona’s traits. TikTok responds most sensitively to a single signal — how long a user lingers over a video. It starts by showing new users very popular items, and sees which catch their eyes.