Blackmail material gathered from it.
Various forms of location data gathered from it, e.g. the location of any member of the military that uses it.
Launching pad for zero days if China feels the need to root someones (or many someones) phones.
Massive social graph, massive amounts of behavioral data. Useful, for doing things like identifying who was likely recruited by a spy agency or identifying potential assets that can be turned.
Potential to pressure speech that China prefers, e.g. by rewarding popular creators with money only if they don't say things China likes, or by decreasing the reach of people who say things China dislikes.
Potential for foreign control of the general trends of content people see. E.g. tweaking the algorithm so "it just happens" to suppress the potential to suppress topics china would prefer we didn't talk about (e.g. their "re-education" camps) or to boost topics they would prefer we did (likely things like political infighting).
Most of these involve various levels of overtness. Many of them it probably only makes sense to start doing once you've captured the market (right now it's more important to onboard more people than to push a message). But I would view all of them as real threats if I was the US government.