if "we were serious" about what? the issue of foreign control is not relevant to domestic companies. we could have some other regulations too, sure, but this one is reasonable.
I think people should be able to decide which social media apps they want to use. They're not even close to reaching the levels of the "infrastructure" box you're forcing them into to justify this decision.
TikTok isn't "infrastructure", TikTok is software. TikTok exploits the infrastructure of the internet across the world, it is not infrastructure itself. The servers TikTok runs on is technically "infrastrucutre", but those same servers could run anything else, the hardware is not "TikTok". I could run "TikTok" the software on any hardware, even if it isn't connected to the public internet, and that would not qualify it as "infrastructure", at least not in the sense that it's servicing any population.