I think they (meaning US government officials) started to care a lot when they realized a non-American social media platform had significant attention from a significant majority of Americans, especially young Americans.

Prior to that, most, if not all, social media platforms were American run, which allowed Congress to have more influence on how they were run and more power to investigate and learn how they were run, compared to one owned by a foreign country, especially one we haven't considered to be a close ally.