Thousands of people said they were experiencing problems logging onto the website and app, loading posts or sending messages, although others reported no problems, APA reported citing Evening Standard.
According to UK site DownDetector, which collects reports of website crashes, the problems on Facebook began at around 2.15pm on Saturday.
People were unable to log onto the website for around an hour, while even more people reported problems loading photo site Instagram, which is owned by Facebook.
The reason of the problem is unknown.