The correct way to reason about this is stochastically. Facebook/unchecked mass communication increases the rate and chance that resentment turns into a mob or violent action. It's rather meaningless to debate the causality of individual cases, but more important to look at the effect in a broad sense.
Also the question to your answer is likely both, as in there's a feedback loop between real world violence, causing more conflict on social media, fuelling yet more violence.
Also the question to your answer is likely both, as in there's a feedback loop between real world violence, causing more conflict on social media, fuelling yet more violence.