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I dont think Cam. Analytica is related to this. I firmly remember it as some code change at Facebooks side as they wanted to monetize groups (sponsored posts). It was not possible to "go viral" on Facebook anymore in a natural way. They capped it somehow. I worked at a newspaper at the time and the change was very clear.

At about the same time these "Bod Dylan has died" fake click baits raged at Facebook, and somehow they stopped. It is probably also related.