In the case of cigarettes, the companies had the answer to these kinds of questions for decades and used it specifically to mislead the direction of public discovery. In their case, they had to risk their own studies in almost all cases instead of just monitoring their own distribution system.

When Facebook says they don't know something there are a few possibilities, the least likely is that they don't know and don't have a reason not to find out.

I do agree with you that it is not up to them to fix. They should be limited to whatever extent possible in what research they can do on their own discretion to prevent them from staying ahead of public information and misleading research.