Mark Zuckerberg (left) and Billie Joe Armstrong
Mark Zuckerberg may have had an intimate, backyard wedding
, but there was at least one famous name on the modest guest list.
Green Day's Billie Joe Armstrong performed at Zuckerberg's Palo Alto, Calif., nuptials on Saturday, PEOPLE has learned exclusively.
"They've been friends for some time," a source close to the artist tells PEOPLE of Zuckerberg and Armstrong. "He wasn't paid. It was a total impromptu wedding present for a friend."
But it wasn't the only surprise of the day.
Though newlyweds Zuckerberg, 28, and wife Priscilla Chan
had been planning their nuptials for more than four months, the ceremony came as a shock to friends, who thought they were coming over for a surprise party in celebration of Chan's UCSF medical school graduation.
The couple, who met while attending Harvard before Zuckerberg – who is now worth more than $19 billion – started Facebook, have been dating for nine years.