Some of Facebook, Instagram and WhatsApp's more than 2 billion users were unable to access the online services from Wednesday until Thursday afternoon as the social network and its sister apps suffered a prolonged outage.
"We are still investigating the overall impact of this issue, including the possibility of refunds for advertisers".
Downdetector.com, a crowd-sourcing site that tracks internet outages, showed a concentration of Facebook outages in the northeast as of Thursday morning.
Pointing out that the Facebook outage also extended to its other properties, including Instagram and WhatsApp, Sarah Miller, the deputy director at the Open Markets Institute, which is for the break-up of Facebook and other Silicon Valley companies, said, "The more integrated a platform becomes, the greater the risk and reach of epic failures like this one".
Facebook takes tens of millions of dollars of advertising revenue every day.
The outage appeared to be abating early Thursday.
Facebook's stock fell almost 3% in early morning trading on Thursday as Wall Street reacted to the outage.
According to the BBC, the outage is the worst in Facebook's history. Reuters was not immediately able to verify those claims and the company declined to comment beyond the statement on resumption of services. Amazon was reportedly allowed to see user contact information.
The outages started around midday Wednesday, hitting users for more than 24 hours and surpassing the tech giant's last comparable outage in 2008. The federal investigation is the latest bad news coming from the company's data privacy scandals.
A spokesman for the social network said the company was cooperating with investigators in multiple federal probes, without addressing the grand jury inquiry specifically.