While investigating the recent theft, the company learned that its Web site had previously been hacked.
"We're assuming it is a large number. It could easily be in the millions," Iannuzzi said in an interview with Reuters.
To be safe, he said, each Monster.com user should assume that his or her contact information has been taken.
The company said earlier that thewas not an isolated incident, and said the scope of illegal activity was impossible to pinpoint.
Monster is stepping up surveillance of site traffic, boosting its security staff, and is contacting users about ways to protect their privacy.
"I want to be clear and I want to be frank: there is no guaranteed fix," Iannuzzi said. "I wish I could say...there will be absolutely no way that
So far, the company has found no evidence that thewere able to take financial information, he said.
About 200 to 300 job seekers have canceled their accounts as a result of the security issue, the CEO said, but those have been offset by an upswing in new accounts. A "handful" of employers have canceled their accounts, Iannuzzi said.
The company, which said last month it would invest $80 million to $100 million over 18 months to improve its technology, will dedicate "a large measure of that money" to fixing the security issue, Iannuzzi said.
Monster shares closed Wednesday up $1.24, or 3.8 percent, to $34.15 on the Nasdaq. Its shares were little changed in extended trading.