I don’t think anyone realized the full extent of the consequences until very recently. Even now, we’re still learning just how pervasive the mental health consequences of social media have been. See the recently released CDC data on teen mental health.
The trajectory for Facebook has been consistently bad. Not knowing the "full extent" is not absolution.
The CDC study is a lagging indicator; it describes the trendline of a mental health crisis that's self-evident to practitioners. On the research side, there have been many studies in the last 10 years associating Facebook use with poor mental health outcomes.
The Cambridge Analytica scandal broke out 5 years ago.