To become one of the dominant digital advertising platforms, Facebook used the data of its members to create hyper-specific categories so marketers could get close to their most valued customers.

That same targeting technology presented a gold mine for Russians over the past five years in their efforts to influence American voters.

In a report created for the Senate Intelligence Committee and provided to CNBC on Monday, researchers analyzed Russia's use of online propaganda to spread false information and polarize U.S. voters between 2013 and 2018. Russia's Internet Research Agency (IRA) reached tens of millions of users in the U.S. on social media, including more than 30 million users who shared IRA-created Facebook and Instagram posts with their circles.

"Indeed, this effective impression management—and fine-grained control over who receives which messages—is what makes social media platforms so attractive to advertisers, but also to political and foreign operatives," the researchers wrote in the report.

More than $1 out of ever $5 spent on online ads will go to Facebook in 2018, according to eMarketer, and the platform is second to Google in the digital advertising market. Advertisers want to be very specific when they show ads to people online because they have to weed through so much content.