To understand this you need to understand that:
1. This is massive with the tween/teen market. That is why not many people understand its popularity; you either need to know a tween, or have one as a kid.
2. Musical.ly became Vine after Vine shutdown. The content moved from lip-synced videos to more of a mixure of vine-like videos and lip sync.
As of now Musical.ly is still #58 on the app store. Its hard to say people aren't using it when its above some large companies on the charts (Linkedin, Shazam, Dropbox).
Their influencer network is huge as well. Nearly every popular musical.ly star has some sort of swag from the company. Think about that in the tween's mind: you're just a normal kid going to your normal middle school with your normal friends, then a national company wants YOU to be a spokesman for the company. That is quite literally getting famous for these kids.
