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NEW YORK (AP) — Facebook's stock plunged to a new low Thursday after the expiration of a ban that had prevented some early investors and insiders from dumping millions of additional shares they own in the social-networking leader.

Firms ranging from Accel Partners to Goldman Sachs, Zynga CEO Mark Pincus and Facebook board members James Breyer, Peter Thiel and Reid Hoffman were among those free to sell stock they own, after the lifting of a ban known as a lock-up period. If many of them took advantage of that, Facebook's stock could decline because the market would be flooded with nearly two-thirds more shares.

It's not yet known whether any of those investors had sold any shares. The stock price decline could have reflected investors' anticipation of such a move.

On Tuesday, shares of online reviews service Angie's List suffered the biggest one-day drop and closed at a new low following the expiration of a similar ban. The price dropped, even though there was no word on whether any of the major investors had dumped their shares.

Facebook Inc.'s stock traded as low as $19.69 before bouncing back to $19.91 in midday trading Thursday. That's still 6 percent down, or $1.29, from Wednesday's close and about 48 percent below its initial public offering price of $38. If the stock hits $19, it will have lost half its value since Facebook went public in May.

By noon, nearly 100 million shares had traded — more than three times the average volume on a full day.

It's been a rough run for Facebook. After one of the most-anticipated IPOs in history, Facebook suffered what may be the most-botched public offering as trading glitches marred its first day. It's been almost all downhill for the Menlo Park., Calif., company since then.

Investors have been concerned about Facebook's ability to keep increasing revenue and make money from its growing mobile audience, even as many analysts hold positive long-term views.

Those eligible to sell stock on Thursday were the investors and directors who had participated in the May IPO. The exception was CEO Mark Zuckerberg, who will be ineligible until November. Microsoft Corp., an early Facebook investor, was eligible to sell, though it was unlikely to do so because of partnerships it has with the social network.

Lock-up periods prevent insiders from unloading shares too close to an IPO and can help prevent volatility that might occur if too many shareholders decide to sell a newly traded stock all at once. They generally start to expire 90 days after a stock makes its public debut. Thursday marked 90 days since Facebook's began trading publicly on May 18.

Other shareholders, including many Facebook employees, will be able to sell beginning in October. The last lockup period expires next May, a year after the IPO.

In all, up to 1.91 billion more shares could flood the stock market over the next several months — more than four times the 421 million shares that have been trading since Facebook's IPO. Of the 1.91 billion, 271 million shares became eligible for sale Thursday.

NEW YORK (AP) — Stocks fell Monday as worries flared anew about the European debt crisis.

The Dow Jones industrial average was down 14 points at 13,261 at noon EDT. The Standard & Poor's 500 was off two points at 1,416. And the Nasdaq composite index was down almost six points at 3,071.

In a report issued Monday, the German central bank questioned the wisdom of having the European Central Bank buy bonds to help the struggling European economies. It stressed that such purchases could carry "substantial risks."

Earlier this month, stocks rallied after ECB President Mario Draghi said the bank might buy government bonds of struggling European countries to help lower their borrowing costs.

In the U.S., Lowe's, the world's No. 2 home improvement store, missed earnings expectations Monday. Revenue at stores open at least a year dipped 0.4 percent, and the company cut its outlook for the year. The stock fell 6 percent.

The most valuable company in the world, Apple, became the most valuable company of all time, with a market value of $621 billion, surpassing Microsoft's record from 1999. Apple stock rose $15.40, or 2.4 percent, to $663.51.

Stocks have been inching up for six weeks. On Friday, both the Dow and the S&P closed just below four-year highs.

Other stocks moving sharply early Monday included health insurer Aetna, which announced it would buy Coventry Health Care for $5.7 billion as the insurance industry realigns itself to better navigate the health care overhaul.

Aetna rose $1.47, or 4 percent, to $39.51. Coventry climbed $6.61, or 19 percent, to $41.55.

The deal follows the $4.46 billion buyout last month of another insurer by WellPoint Inc., and last year's acquisition worth nearly $4 billion by Cigna of HealthSpring as it grabbed for a share of Medicare revenue.

Best Buy slid 7 percent after rejecting an offer from its founder and largest shareholder to take the electronics retailer private. The company named Hubert Joly, the former head of global hospitality company Carlson and a turnaround expert, as CEO Monday.

Facebook gained 36 cents, nearly 2 percent, to $19.41, following a slide last week after some insiders were able to sell stock for the first time since the company's public trading debut in May. The stock is down about half from its offering price of $38.

In the S&P 500, seven of the 10 main industry groups fell, led by telecommunications and consumer discretionary stocks. Those were down more than 0.5 percent each.

In Europe, stocks were also falling. Greek stocks fell 2 percent. Spain's main index was off 1 percent.

After the ECB's Draghi raised the possibility of bond-buying, German Chancellor Angela Merkel, who has resisted easing pressure on struggling countries, made comments that seemed to seem soften her resistance. She said last week that officials would do everything possible to save the euro.

But now investors are not sure the ECB will come to the rescue.

Investors are on edge this week because of a series of meetings among European leaders to discuss the debt crisis. The first of them came early Monday when the Greek foreign minister met with his German counterpart in Berlin to discuss Greek spending cuts necessary for the country to continue receiving bailout money.