TT's addictive nature is more insidious because it's more internal: Humans are pattern-matching machines and TikTok will show you a LOT of very niche content to the point where your brain thinks it's important and true (think TV/movie product placement; they wouldn't do it if it didn't work). Because of TT's form, it's harder for the average person (non-creator) to be drawn into a social group, but it's easier for them to be firehosed into accepting false narratives through sheer repetition. TT distorts a person's idea of HOW PEOPLE LIVE; imagine if your main idea of how Americans live was USSR propaganda. TT makes people apathetic and prone to hug-boxes.
They're both taking advantage of the fight-or-flight mechanism, FB just wants you to fight and TT wants you to 'flee' (into its loving escapist arms, of course).
There are healthy use cases for both FB and TT (and even Twitter), but they're directly at odds with the incentives of the platforms themselves + we have a society full of unhealthy and traumatized people, so instead you get these evil clone versions.
I'd also say that both the US and China are authoritarian countries. China more so, but the mental health/addictive problems with TT are inherent to its design and medium decisions, not which nationality made it.