Musical.ly just sold to Beijing Bytedance Technology, the companies announced Friday. The price tag was between $800 million to $1 billion. If you haven’t heard of Musical.ly, you may be old — or perhaps you just forgot about it. After launching three years ago, the smartphone app quickly became a viral sensation. The concept was simple: Create 15-second music videos. Tweens and teens became obsessed with lip-syncing to their favorite songs. Musical.ly was seen as another form of social media and short-form creation, competing with Twitter’s Vine, Facebook’s Instagram, and Snapchat. Its audience skewed young. More than 60 percent of its users were between 13 and 20 years old. Last week could we also read that Tencent purchased 10% of Snap Inc.