That accounts for nearly 20% of all time online.
"If time is money, then the Facebook.com site represents the most valuable Internet property on the web today," Needham analysts write in their research note.
Although 20+ minutes is the global average, people in the US spend much more time than that, according to Facebook's internal numbers.
In Facebook's Q2 earnings last year, CEO Mark Zuckerberg said that the average US consumer spends 40 minutes on Facebook per day.
It will be interesting to see whether the company gives any update on that stat when it announces its earnings at the end of the month: Needham thinks that people are experiencing "Facebook fatigue."
About 30% of the US respondents in its proprietary survey said they were using Facebook less today than one year ago.