The hackers accessed name, email addresses or phone numbers from those 29 million accounts.
For 14 million of those accounts, hackers got even more data, such as hometown, birthdate, the last 10 places they checked into or 15 most recent searches. One million accounts were affected but hackers didn't gain information. The social media service plans to send messages to people whose accounts were hacked.
Facebook says third-party apps and Facebook apps like WhatsApp and Instagram were unaffected by the breach.
Guy Rosen, Facebook's VP of Product Management, took to the social media giant's blog to discuss recent security problems on the site.
"We have been working around the clock to investigate the security issue we discovered and fixed two weeks ago so we can help people understand what information the attackers may have accessed," he wrote. He also noted the company has "not ruled out the possibility of smaller-scale attacks," and will continue to investigate.
"In the coming days, we’ll send customized messages to the 30 million people affected to explain what information the attackers might have accessed, as well as steps they can take to help protect themselves, including from suspicious emails, text messages, or calls," Rosen said.
Those who are worried about their account's security can visit Facebook's Help Center. To read Rosen's full blog post, click here.