Responsibility is not zero sum. The people who pulled the triggers? Responsible. The people who gave those people the orders? Responsible. People who told them where to find the people they killed? Responsible. Arms dealers who sold them the guns? If they had any inkling that this was a possible outcome, then they share in the responsibility. The people behind the legacy media efforts that whipped people into a frenzy? Responsible. And so on.
Facebook is not like a screwdriver, a simple, neutral tool that is occasionally used to harm. Facebook is an incredibly complex tool for connecting humans, a tool with all sorts of biases that shape the nature and velocity of social interaction.
People have known for decades that online communication systems enable harm. This was a well-understood fact long before Facebook existed. Facebook is morally responsible for that harm (as are the perps, etc, etc). Something they understand perfectly well because they do a lot to mitigate that harm while crowing about how socially responsible they are being.
You might disagree with most ethicists on this, as well as with Facebook itself. But you'll have an uphill struggle. Even the example you pick, vehicles, doesn't work, because car manufacturers have spent decades working to mitigate opportunities for the tools they create to cause harm. Now that cars are getting smarter, that harm reduction will include preventing drivers from running the cars into people.