Billions of downloads doesn't tell you if 'something is growing' it's the Daily Active Users. Ask them how TikTok plans to make money in order to remain profitable and they will completely fall tone deaf.
This article tells you nothing about TikTok outperforming instagram.
Now what the retention looks like, that’s another story.
That's what's got me skeptical. There's a lot of people saying TikTok proves Twitter killing Vine was a bad move, but Facebook, Instagram, and WhatsApp are wildly different "social networks". IMO the equivalent of putting Amazon, Youtube and Wikipedia on the same "website" category.
WhatsApp is incredibly useful. It's perhaps the "lamest" of all social networks— it doesn't have "creators", it's not known for its cool filters, it has no "likes", but it works and where it's used, it's as ubiquitous and used as phone numbers once were.
Facebook is very utilitarian too. IMO they haven't become "uncool" because they dropped the ball— they went for the general population. It's like they purposefully toned down their "cool" brand to widen the age of users that could associate with it. Now you can not only find pretty much anyone on Facebook, but also buy and sell on Marketplace, find a Job, find events, restaurants, find groups for parents, study groups, groups for people with rare diseases, the list goes on.
TikTok is very popular, but what's the demographic and what's the value proposition here? Teenage entertainment? If so, they're competing with Instagram, but also YouTube and Netflix. It's also perfectly plausible its users will outgrow it by the time they go to college or enter the workforce, or once the ads pick up to keep with the operational expenses.
The real challenge for the company will come in the form or retention and the broadening of their product + user base to compete on the next level, as something useful and not just entertaining, IMO.