The main Facebook Web domain wouldn't load, and photo-sharing service Instagram was also unusable.
It's not immediately clear what is causing the problem, or how long the outage might last. The issues affected users in Asia, Europe and the U.S., according to the site. "Thank you Zuckerburg, great service" said another.
Facebook didn't returned emails seeking comment on the outage.
The massive outage comes about a month after a similar widespread outage shut down Facebook's services.
The sites were back in business around 9 a.m. EST.
"We're aware that some people are now having trouble accessing the Facebook family of apps. We are investigating the issue and working on a resolution", Facebook wrote, adding that it was "investigating".