The big issue isn't data security; it's propaganda. Irrespective of whether the government has control of the narrative on Facebook (I would argue they pretty clearly don't) there is no reason to let a foreign adversary have a deniable propaganda line to millions of Americans. Would we have let the USSR acquire a major television network?
And even if you disagree with the national security reasons for disallowing China to control a major U.S. social network, there is still the issue of trade reciprocity - nearly all of the U.S. Web companies are banned in China.
And even if you disagree with the national security reasons for disallowing China to control a major U.S. social network, there is still the issue of trade reciprocity - nearly all of the U.S. Web companies are banned in China.