Why is fighting online extremism so hard?

Following Saturday's attack in London, Facebook is promising to do more to keep terrorists off its network. But stopping extremists from spreading their message of hate across the web is so difficult because it requires vigilance and cooperation from law enforcement, internet users, and most importantly social media companies, experts told Fox 5. While Facebook and others say they already work to combat online extremism, some critics said that is not nearly enough.

"We need to work with allied democratic governments to reach international agreements that regulate cyberspace to prevent the spread of extremist and terrorism planning," Prime Minister Theresa May of the United Kingdom said Sunday. "And we need to do everything we can at home to reduce the risk of extremism online."

Prime Minister May demanded more help to fight terrorism from internet companies. Counterterrorism officers allegedly recorded one suspect from the extremist cell responsible for Saturday's attack discussing how to use YouTube videos to plot a van and knife attack in London.

Facebook said it already does what the prime minister is asking and that it does not allow groups or people that engage in terrorist activity to post on the site.

"We want Facebook to be a hostile environment for terrorists," Facebook said in a statement.

Twitter has suspended hundreds of thousands of profiles related to promotion of terrorism. The company is also condemning terrorist content.

"We will never stop working to stay one step ahead," Twitter said in a statement.

"One of the easy things that can be done now is stopping a lot of the propaganda material that's basically proliferating on a lot of these platforms," Tara Maller, a cyber security expert told Fox Business. "That's basically how a lot of these individuals come into contact with ISIS material or terrorist propaganda."

Maller and Dartmouth College Professor Hany Farid work on the Counter Extremism Project. Farid is skeptical that without a financial incentive social media companies won't take an aggressive enough approach. Facebook has said it is going to hire more people to monitor content.

"These extremist groups have weaponized the internet and they are using it to great effect," Farid said. "Facebook and Twitter and YouTube are sitting on the sidelines and doing nothing but making boatloads of money off of it and I think it's inexcusable."

Facebook, Microsoft, Google, and Twitter teamed up last year to create a database of unique digital fingerprints for images and videos that are produced by or support extremist organizations. Those fingerprints help the companies identify and remove extremist content.