Facebook shares sank in the first day of trading without the full support of the company's underwriters, leaving some investors down almost 25 percent from where they were Friday and driving others to switch back to more established stocks.

Facebook's debut was beset by problems, so much so that Nasdaq said on Monday it was changing its IPO procedures. That may comfort companies considering a listing, but does it little for Facebook, whose lead underwriter, Morgan Stanley, had to step in and defend the $38 offering price on the open market.

Even so, one source said Morgan Stanley's own brokers were at one point "ranting and raving" about glitches that left unclear what trades had actually been executed.

Without a fresh round of defense, Facebook shares fell $3.79 to $34.34 in afternoon trading. That represented a decline of 10 percent from Friday's close and 24 percent from Friday's intra-day high of $45 a share.