How Facebook Security Attack Affected 50 Million Users

A hacker gained access to nearly 50 million Facebook user accounts by exploiting a weakness in the social network's systems, Facebook said on Friday.

News of the cyber attack — which appears to be one of the most significant in Facebook's history — sent shares of the company down roughly 3% in midday trading on Friday, adding to the pile of woes currently weighing on the company.

Facebook CEO Mark Zuckerberg hosted a conference call with journalists shortly after the news was announced, underscoring the severity of the situation.

"We do not yet know whether these accounts were misused but we are continuing to look into this and will update when we learn more," Zuckerberg said in a blog post published on Friday.