In California, for example, nearly 24 percent of online voter registrations during the Facebook campaign came from residents aged 17 to 25. Nearly 30 percent more came from Californians from 26 to 35 years old.

“It’s pretty clear that the Facebook reminder campaign disproportionately motivated young people to register,” Mr. Padilla, the secretary of state, said.

The reminder — “Are you registered to vote? Register now to make sure you have a voice in the election.” — was presented alongside two links: one leading to a federal directory of state voter registration websites; and another allowing users to share that they had registered. Only users who would be of voting age on Election Day saw the reminder, which appeared for both desktop and mobile users.

Officials greeted the effort enthusiastically.

“I applaud Facebook for joining our efforts to increase voter registration awareness,” Brian Kemp, Georgia’s secretary of state, said in a September statement, noting a substantial increase in traffic to the state’s voter registration website.

In Connecticut, 14,883 people registered to vote in the first three days of the Facebook campaign, a more than 12-fold increase over the same period a week earlier, the secretary of state said in a statement.

The secretaries of state in Colorado, Kentucky, Nebraska and West Virginia also credited Facebook, at least in part, for boosting registration.

While the reminder was part of an inaugural national campaign for the social network, it built on a tradition of promoting civic engagement for the company. As it has since 2008, Facebook plans to remind users to vote on Election Day.