Terrorists and hate groups continue to get their propaganda onto social media platforms despite efforts by Facebook, Twitter, and YouTube to shut them down, a US Senate panel heard on Wednesday.

Islamic State, Al Qaeda, and others have stepped up their use of bots and other methods to fight the artificial intelligence and algorithms the social media giants deploy to screen them out.

In addition, they are now turning to smaller platforms and messaging apps with encryption and less ability to police users, like Telegram, Reddit and WhatsApp, though none have offered yet the previous broad reach that Facebook and YouTube have had.

Nevertheless, the largest social media firms were pressed in a Senate Commerce Committee hearing Wednesday over their reliance on artificial intelligence and algorithms to keep their powerful platforms clear of violent extremist posts.

A key concern is the continued ability to use anonymous accounts, which while benefiting pro-democracy activists battling repressive governments, also continue to empower extremists.

"These platforms have created a new and stunningly effective way for nefarious actors to attack and to harm," said Senator Ben Nelson, who said that crackdown efforts by the social media giants so far are "not enough."

YouTube is automatically removing 98 percent of videos promoting violent extremism using algorithms, said Public Policy Director Juniper Downs.

Facebook's head of Product Policy and Counterterrorism, Monika Bickert, said that 99 percent Islamic State and Al Qaeda-related terror content "is detected and removed before anyone in our community reports it, and in some cases, before it goes live on the site."

Senator John Thune, Chairman of the Commerce Committee, countered by asking why a video showing how to build a bomb -- which was used by the man who attacked the Manchester Arena in June 2017 -- has repeatedly been uploaded to its website every time YouTube deletes it, as recently as this month.

"We are catching re-uploads of this video quickly and removing it as soon as those uploads are detected," said Downs.

Carlos Monje, director of Public Policy and Philanthropy for Twitter, said that even with all their efforts to fight terror- and hate-related content, "It is a cat-and-mouse game and we are constantly evolving to face the challenge."

Clint Watts, an expert at the Foreign Policy Research Institute in the use of the internet by terror groups, testified that the efforts by the social media companies have been quite successful, but are still missing significant amounts of unwanted postings.

"Social media companies continue to get beat in part because they rely too heavily on technologists and technical detection to catch bad actors," said Watts.

"Artificial intelligence and machine learning will greatly assist in cleaning up nefarious activity, but will for the near future fail to detect that which hasn't been seen before."

Last year Google, Facebook, Twitter and Microsoft banded together to share information on groups and posts related to violent extremism, and to share techniques on keeping it off their sites.

All are struggling with the problem of anonymous accounts or accounts with fake owners. Watts called that the "most pressing challenge."

"Anonymity of social media accounts has in many cases allowed the oppressed and the downtrodden to speak out about injustice," Watts said.

"But over time, anonymity has empowered hackers, extremists and authoritarians to inflict harm on the public."

Monje illustrated the problem: Twitter, he said, believes that less than five percent of its 300 million accounts are fake.

But, he said, "They keep coming back... We are now challenging four million malicious automated accounts a week."

Watts said extremist groups have nevertheless been frustrated by the effort to censor them on social media and are actively searching for new outlets.

"They are looking for a place where they can communicate and organize. They have to be able to push their propaganda globally in order to recruit," he told the Senate panel.

"They are seeking a new home but they just haven't found it yet."