Likely referring to this article [1].
However, the claim that they made "hundreds of thousands of people fall into a depression" is pretty exaggerated - FB demonstrated it could influence the mood of users via changes to the algorithm, but the overall effect size was small (Cohen's d = 0.001).
I am very critical of FB and this type of research, but don't know if anyone became depressed over the experiment. It's possible, but wasn't demonstrated.