Nothing honestly, but FB/Google/Twitter are under the authority of the United States government. Thus it is less of a threat to the US government, which is what it means to be a national security threat in this context.
The same way we figure out anything else - learning and experience. Ultimately, there have been a bunch of cases of people like us having their lives ruined by the USG, and not so many cases of the CCP doing the same (and none for people who didn't go to China or have family ties there, AFAIK).
> There is evidence of foreign governments meddling with US civilian affairs. It seems like this is exactly the attitude they would try to cultivate.
There is also evidence of the US government meddling with US civilian affairs, and I suspect they deploy a lot more and a lot more effective propaganda to cultivate the attitudes that they want to see.
If the data is dangerous, no one should be allowed to collect, store and sell it. It shouldn't be allowed for only those in the favor of the US federal govt.