Finding audio and digital forensic experts who could verify that the video had been manipulated required time, but while the fact-checkers were investigating the case, misinformation continued to spread at Internet speeds.
Facebook works only with fact-checkers that are part of the International Fact-Checking Network (IFCN), CNN reported on Saturday.
When one of Facebook's fact-checking partners rates a post or video as false on the platform, it automatically triggers a change in how Facebook's algorithm handles that content.
Demoting a piece of content would let it appear on the platform, but, less frequently in users' news feeds.
It also notifies Facebook users who share or have shared the content that it's false, the report quoted Facebook as saying.
For now, the tampered video of Pelosi remains on Facebook, but the social networking giant is alerting people who are tying to re-post the video on their timeline.
"Once the video was fact-checked as false, we dramatically reduced its distribution," The Verge quoted a Facebook spokesperson as saying.
"Speed is critical to this system, and we continue to improve our response. People who see the video in feed, try to share it from feed, or have already shared it are alerted that it's false," the spokesperson said.
The video was first posted on a page called Politics Watchdog, but it didn't mention that the video was edited in any way for any purpose. On this page alone, Pelosi's video gathered over 2.5 million views.
The 79-year-old Speaker of the House of Representatives has often been targeted by US President Donald Trump and his allies.
The viral video in question was also re-shared on Facebook by Trump's lawyer Rudy Giuliani.
( With inputs from IANS )