It became clear really quickly that they have completely disrupted instagram and snap. The UX is much more communal and interactive than either of those, the algorithm lands you on your interests faster, and they have massively reduced the friction around becoming a content creator rather than just a consumer - so there's simply more STUFF there. The videos are more ephemeral and imperfect than what you usually see on IG, and there's a lot more joy in what's being captured. It really is a step change in the ability to build communities around shared interests.
Behind the scenes, it's a privacy nightmare and I understand the national security concerns that go with PRC tapping directly into the id of our electorate. I think PRC is much more skilled and subtle about online manipulation than Russia is. (I was working on media monitoring when social media became strangely fixated on trade deals for just long enough to get US involvement in TPP killed - a framework that was only significantly different from other trade deals in that it would sideline PRC. Once both candidates came out against it, popular concern over trade deal nuances disappeared and never returned)
But the hysteria is driven just as much by SV tech knowing that they've been lapped and finding common ground with congress for purely anti-competitive reasons, as it is by the government trying to thread the needle between free speech and ceding ground to adversary information ops.