According to a CNBC report, anonymous employees broke this news, saying Instagram reached the 2-billion users' figure about a week before Facebook changed its name to Meta in October.
Instagram hasn't made its user numbers public since it crossed the one billion monthly active users mark in June 2018.
It has taken three years for Instagram to hit the two billion-mark.
The Instagram employees reportedly said they found out about the number during internal conversations.
Last week, testifying for the first time at a US Senate hearing in Washington, D.C., Instagram CEO Adam Mosseri defended the platform's impacts on teens, saying that the photo-sharing platform "quite frankly wasn't designed" for people between the ages of 13 and 17.
The hearing came after leaked internal research at Meta showed that the photo-sharing app could harm its young users.
Mosseri said that "respectfully, I don't believe that research suggests that our products are addictive".
"We know that 10- to 12-year-olds are online. We know that they want to be on platforms like Instagram. And Instagram quite frankly wasn't designed for them," he said during the hearing.
Meta has been grilled recently after a leak exposed how Instagram's own research had found the platform could harm children's well-being.
A global coalition of researchers has called on Meta to be more transparent and serious about the mental health of child and adolescent users on Facebook, Instagram and WhatsApp, as debate rises over the harmful impact these platforms have on the minds of children.
Facebook whistleblower Frances Haugen had testified before the US Congress that Instagram can have a negative effect on the mental health of teenagers.
The Meta-owned photo-sharing platform last week launched a 'Take a Break' and other safety features for teenagers.
In late September, Mosseri had announced that Instagram would pause its plans to develop Instagram Kids, a version of the app specifically for children under 13.