"Yesterday, we made a server configuration change that triggered a cascading series of issues. We are 100 percent back up and running and apologize for any inconvenience", a Facebook spokesperson said.
The outage-tracking website Down Detector showed significantly fewer people having issues accessing Facebook and Instagram on Thursday as compared to Wednesday, when the outage began.
Facebook took to its Twitter account Wednesday to assure users that the outage wasn't due to a "distributed denial of service" attack, or DDoS, meaning the disturbance wasn't the result of hacker attacks. The company did not immediately respond to a request for comment.
Facebook experienced a similar outage in 2008 when it had 150 million users. Facebook's desktop platforms and apps were affected in parts of the USA, including the East and West Coast, as well as parts of Europe and elsewhere.
Separately, the New York Times reported on Wednesday that US federal prosecutors were conducting a criminal investigation into data deals Facebook struck with more than 150 technology companies such as Amazon.com Inc and Apple Inc.
According to the report, a grand jury in NY has subpoenaed information from at least two major smartphone makers about such arrangements with Facebook.
The social network's handling of user data has been a flashpoint for controversy since it admitted previous year that Cambridge Analytica, a political consultancy which did work for Donald Trump's 2016 election campaign, used an app that may have hijacked the private details of 87 million users.
The news comes with regulators, investigators and elected officials in the U.S. and elsewhere in the world digging into the data sharing practices of Facebook. Facebook said it was working to resolve the issue but didn't announce the fix until Thursday.