"We're aware that some people are now having trouble accessing the Facebook family of apps", Facebook tweeted.
It further continued that Facebook is working to resolve the issue as soon as possible.
Facebook and Instagram are partially down for many users around the world on Wednesday.
When attempting to log in to the site, some users were greeted by a message stating "Facebook will be back soon".
Are you experiencing problems with Facebook and Instagram today?
A dedicated denial of service attack, or DDoS attack, is when someone floods a website with fake traffic causing the site's performance to suffer when there are not enough resources for real traffic.
The Verge tested Facebook's ad platform and found problems there, too.
The website Down Detector showed a surge in complaints today with users in Europe, the United States and Asia among the worst-hit.
Facebook-owned apps Instagram and WhatsApp also were experiencing performance issues, according to some users.
The outage includes Oculus, Facebook's virtual reality company and Workplace, its corporate comunications software.
While desktop versions aren't even loading for many users, apps do load the content, but you can't post new status updates or send / receive messages. Many report being unable to post, share, or like. That outage was the second time that month.