The digital army sprung to life with a click of a mouse in a nondescript office park in Alexandria. Less than 10 miles away, at the White House, the phones began to light up. One call came into the switchboard and then another. Thousands of people flooded the phone lines.

It was early August 2014, and the callers were conservatives lambasting President Obama for promising what they described as "executive amnesty." The deluge of angry activists was not the work of a heavily coordinated national campaign, a pricey phone-banking operation, or really an exhaustive effort of any kind.

It resulted from a single post on Facebook.

The volume of calls was so high that, within hours, the White House complained it was a "security issue," according to an email from the phone vendor hired to connect callers to the switchboard. More than 9,000 calls had been made before they pulled the plug. At the headquarters of ForAmerica, the conservative group that had launched the telephone broadside, the White House's reaction was seen more as victory than defeat.

"We got our point across," said David Bozell, ForAmerica's executive director.

In the last four years, ForAmerica has quietly amassed what it likes to call a "digital army" on Facebook—a force that that now numbers more than 7 million. The group's spectacular growth can be explained in part by the paid acquisition of its members through targeted advertising. But thanks to a daily stream of savvy and snackable red-meat messaging, these mercenaries have become loyal conservative digital soldiers whose engagement is attracting new recruits. These days, a routine post on ForAmerica's page reaches more than 2 million people, achieves more than 100,000 "likes," and has tens of thousands of people repost and comment.