PITTSBURGH — When thousands of Trump supporters, in an estimated 2,000 cars and trucks, gathered for the “Interstate 70 Trump Train,” a road rally covering Ohio, West Virginia, and Pennsylvania last weekend, I asked many of them how they had learned about the event. Nearly every person said they heard about it on Facebook. That was especially important, they added, because there was little or no news coverage of such events.

This weekend, there will be another giant car and truck rally, the “Pro-Trump PA Turnpike Relay Parade,” this one across the highway that joins eastern and western Pennsylvania. I asked the organizer how he is getting the word out. The answer: Facebook.