The site, which has more than 160 million members, still says it has yet to determine the full extent of the breach several days after news of the theft of the passwords emerged.
Some cyber security experts say LinkedIn did not have adequate protections in place, and warn that the company could uncover further data losses over the coming days as it tries to figure out what happened.
LinkedIn is conducting an investigation to determine how more than six million customer passwords turned up on underground sites frequented by criminal hackers. A company spokesman, Hani Durzy, said LinkedIn does not even know if any account information was stolen besides passwords.
The dearth of information has left some security professionals and customers worried that LinkedIn's computer systems may have suffered a more serious breach.
"There is going to be more to come," said Jeffrey Carr, chief executive of security firm Taia Global. "As long as they don't know what happened here, there is a good chance that it is more widespread than originally thought."
Customers whose passwords were among those stolen were still getting notified by LinkedIn as of Friday afternoon, days after news of the breach first surfaced.
Laura DiDio, a technology analyst with a consulting firm known as ITIC, said that was not fast enough.
"I am angry," she said. "As soon as there was an inkling that there was a breach, they should have been all over this. I want to know what they are doing to correct this situation."
Some security experts say the company's data security practices were not as sophisticated as one would typically expect from a major Internet company.
For example, they noted that Linked In does not have a chief information officer or chief information security officer.
Those are positions that typically supervise technology operations and computer security at large corporations.
A company spokeswoman, Erin O'Hara, said the company did not have managers with those titles, but that its senior vice-president for operations, David Henke, oversees those functions.
Several experts said the company fell down in the way it encrypted, or scrambled, the passwords that were stored in the database.
The technique they used to encrypt those passwords is a relative simple one that hackers can crack fairly quickly with only a moderate level of skills and widely available computer resources, they said.
When asked to comment on that criticism, the company said on Thursday that LinkedIn was already taking steps to improve security, including improving the technique it uses to protect those passwords.
LinkedIn is a natural target for data thieves because the site stores valuable information about millions of professionals, including well-known business leaders.