Yesterday, fewer than 100 people showed up at Mark Zuckerberg's Palo Alto home.

They thought they were attending a graduation party for Priscilla Chan, Zuckerberg's girlfriend of 9 years who had just finished medical school.

Instead, they found themselves at a wedding, AP reports.

Upon arriving they were told they weren't just party guests, they were wedding guests. Sheryl Sandberg, Facebook's COO, was in attendance.

"Everybody was shocked," a guest told The Associated Press.

The wedding had been planned for months, says this person. But its timing, so close to Facebook's IPO, wasn't planned as that date was more of a "moving target."

Hardly anyone -- aside from those close to the couple -- knew Chan and Zuckerberg were engaged. Last June a rumor was reported that the couple was tying the knot when Bill Gates accidentally called Chan his fiance in The Daily Mail.