Top Facebook executives including Mark Zuckerberg and Sheryl Sandberg were not aware of the specific work being done by Definers, the person said.
In a statement, Facebook said it had not hidden its ties to Definers and disputed that it had asked the firm to spread false information.
"It is wrong to suggest that we have ever asked Definers to pay for or write articles on Facebook's behalf, or communicate anything untrue," a Facebook spokeswoman said in the statement.
"The relationship with Facebook was well known by the media - not least because they have on several occasions sent out invitations to hundreds of journalists about important press calls on our behalf," the spokeswoman added.
Definers was founded by veterans of Republican presidential campaigns and specialised in applying political campaign tactics to corporate public relations. Last year, Tim Miller, a Definers official and former spokesman for Jeb Bush, started a Silicon Valley chapter. He said in one interview that as technology firms mature, a goal should be to "have positive content pushed out about your company and negative content that's being pushed out about your competitor".
Facebook initially hired Definers to monitor news about the social network. It expanded its relationship with the firm in October 2017 when scrutiny of Facebook was increasing over how Russian agents had used the social media site to sow discord before the 2016 US presidential election.
The Times reported on Wednesday that earlier this year, a conservative website called NTK Network began publishing stories defending Facebook and criticising Facebook rivals like Google. NTK is an affiliate of Definers.
In addition, Definers circulated a research document this summer casting Soros, the billionaire liberal donor, as the unacknowledged force behind what appeared to be a broad anti-Facebook movement. Definers pressed reporters to explore the financial connections between Soros and groups that had criticised Facebook, including a progressive group founded by Soros' son and Color of Change, an online racial justice organisation.
An official at Soros' Open Society Foundations said that the philanthropy had supported both member groups but that no grants had been made to support campaigns against Facebook.
"We are proud to have partnered with Facebook over the past year on a range of public affairs services. All of our work is based on publicly available documents and information," a Definers spokesman said in a statement.
He added: "The document referenced in the Times story regarding the anti-Facebook organisation's potential funding sources was entirely factual and based on public records."
Miller said late on Wednesday on Twitter that he was hurt by accusations that Definers' work related to Soros was anti-Semitic. "Im disgusted by the rise of anti-semitism including people who have falsely targeted Soros. It's deeply deeply personal. I've continuously fought the alt-right & others who spread racist lies & hate & will keep doing so," he said.
After the Times article, other organisations also began re-evaluating their relationship with Definers. One of those was Crooked Media, which runs the popular political podcast Pod Save America. Miller is a frequent contributor to the podcast.
"We need to get to the bottom of Tim's involvement in this work, and he won't be contributing to Crooked more in the meantime," Jon Favreau, Jon Lovett and Tommy Vietor, three of the hosts of the podcast, said in a statement.