Zynga shares fell today as fears about the company’s user base prompted another sell-off. Zynga’s shares dropped as much as 13 percent. That triggered a short-sale circuit breaker, which halts trading in the stock until cooler heads prevail, for the first time.

In a research note, Cowen analyst Doug Creutz wrote that Zynga’s daily active users declined by 8.2 percent last month, the second month in a row for such a drop. Zynga’s stock closed today at $4.98 a share, down 10.2 percent from the previous day.

“Nearly all of the company’s major titles declined significantly,” he said.

Zynga’s shares have been as high as $15.91 a share on March 2, driven by speculation that the company might benefit from a relaxation of laws that restrict online gambling. Zynga has the world’s largest online poker game, Zynga Poker, but users can’t play for real-money winnings in that game. But Zynga suffered a big drop after Facebook’s IPO got a tepid reception on the stock market. Zynga is Facebook’s largest revenue generator.

The Nasdaq Stock Market halted Zynga trading at 9:59 am eastern time today, after Zynga’s stock fell. Zynga went public in December at $10 a share.

Zynga has been moving to reduce its dependence on Facebook, which accounts for more than 90 percent of Zynga’s revenues. Zynga has launched Zynga.com. Zynga is also spreading out internationally and moving into mobile games.