Mark Zuckerberg -- Facebook Stock PLUMMETS

Tough morning for The Zuck -- after a lackluster debut ... Facebook stock is sinking like a stone this morning ... down more than 11% from its opening price.

As of 7:32 AM PDT ... FB had fallen to $33.97 per share ... down $4.26 from its closing price on Friday.

According to MSN.com, the stock was down as much as 14% earlier in the morning.

FYI -- Facebook debuted at $38-per-share ... and many hoped it would explode. It didn't.

Of course, we're still a long way from the closing bell ... and a lot could happen -- good or bad.