Terrorists are now using more than 70 online platforms to spread propaganda, a new report has warned.
Islamic State in particular has exploited social media and messaging apps to promote its cause and recruit followers.
Figures released by Europol highlight the volume and breadth of material being pumped out by extremists.
The EU Internet Referral Unit, which was launched a year ago, has identified more than 70 platforms "used by terrorist groups to spread their propaganda materials".
Investigators have assessed and processed in excess of 11,000 pieces of content in eight languages.
Of those, 9,787 were referred to 31 platforms, with 91% of the items removed. The report said: "This is a voluntary arrangement and it is ultimately for the companies to decide whether to remove the material."
Europe has been facing the "most significant terrorist threat of the past decade", according to the report published by Europol on Friday.
It said: "While the so-called digital revolution has empowered citizens in unprecedented ways, technological developments and the emergence of social media have changed the behaviour, interaction, modus operandi and ways of communication of terrorist groups.
"Recent terrorist attacks and thwarted attacks have shed light on how the internet is currently used by terrorists as a key element to disseminate propaganda, recruit, radicalise, co-ordinate terrorist activities and glorify their atrocities. More recently, we have seen how terrorists use the internet during the attack phase as well."