SAN FRANCISCO — A group of Silicon Valley technologists who were early employees at Facebook and Google, alarmed over the ill effects of social networks and smartphones, are banding together to challenge the companies they helped build.

The cohort is creating a union of concerned experts called the Center for Humane Technology. Along with the nonprofit media watchdog group Common Sense Media, it also plans an anti-tech addiction lobbying effort and an ad campaign at 55,000 public schools in the United States.

The campaign, titled The Truth About Tech, will be funded with $7 million from Common Sense and capital raised by the Center for Humane Technology. Common Sense also has $50 million in donated media and airtime from partners including Comcast and DirecTV. It will be aimed at educating students, parents and teachers about the dangers of technology, including the depression that can come from heavy use of social media.

“We were on the inside,” said Tristan Harris, a former in-house ethicist at Google who is heading the new group. “We know what the companies measure. We know how they talk, and we know how the engineering works.”