The "TikTok threat" is so overblown. It's not activity competing with any existing social or video platform. It's competing for time, but that's about it. No one is using TikTok to stay in touch with their Grandmother or watching 2 hour Podcasts on it. They invented a new type of video entertainment which has a social element, that's it - it's neither a YouTube or a social network competitor.

I'd argue Snapchat is probably their most direct competitor, but Snapchat's DAU is still growing fine, because even in Snapchat's case I think you can argue they're used differently. Snapchat seems to be used primarily to stay in touch with friends where as TikTok seems to be used to find/share short-form video content with strangers.