Facebook is down and the cause is unknown.

Thousands of Facebook users this afternoon (U.S. Pacific Time) are reporting that the popular social networking site is down. It’s unclear when the outage began. PCWorld has not been able to reach Facebook for comment, but Mashable reports the company has confirmed the outage.

Facebook fans are using Twitter to voice their frustration.

“Yes, facebook has crashed. It will be fine, now you can spend the rest of the day ACTUALLY doing a full-day’s work,” wrote Twitter user B_Walker.

“Facebook is down? You know, we could always go back to MySpace. (crickets, followed by boos) Yeah, never mind. Don’t do that,” suggested “RyanMConners.”

We’ll update this story as more information becomes available.

UPDATE: Facebook seems to be up and running again – yay!