Social media sites including Facebook, Instagram and WhatsApp are back online after being unavailable for almost six hours last night. 

The outage lasted from around 4pm until 10pm yesterday, with Facebook blaming an internal technical issue for the problem. 

The outage lasted so long because the problem also stopped staff from entering Facebook headquarters as work passes and email stopped working. 

The company has reassured users that there is "no evidence that data was compromised". 

More than 3.5 billion people across the world use Facebook and its sister sites Instagram and WhatsApp.