Workers who had just received their salaries and were busy settling bills online badly hit.
Facebook shares were down 2.3% during pre-market trading at 6.51am in New York Thursday.
As of early Thursday, the problem remained in parts of Europe and Asia, although moaning from America and elsewhere continued.
Ad sales are the company’s lifeblood and persistent difficulties could be costly.
The tech giant says the outage is not related to an attack aimed at overwhelming the network.
Many users are unable to receive emails or download attachments.
Users took to Twitter to complain about the outage and tweeted messages along with the #instagramdown hashtag while Facebook and Instagram were not immediately available to comment.
Most affected users experienced problems for less than 90 minutes and the problem was not specific to a particular region.
The outage, which affected trading and stock exchange releases, began just after 1119 local time (2319 GMT Monday), and remained unresolved throughout the afternoon.
The Philadelphia-based company said two separate, unrelated fibre cuts at backbone providers halted service to business and residential customers.
The month-long Sabah Saidi 100 Lab was launched today to ensure that electricity interruption in minutes per customer is reduced to 100 minutes by 2020.
The first outage was at 6.38pm yesterday and the other at 3.30am today.