An expert in online extremism says terrorist recruiters are "barely present" on Twitter and Facebook these days.
Professor Jytte Klausen, the Lawrence A. Wien Professor of International Cooperation at Brandeis University, is a leading international expert in extremist Islamist ideology.
In 2014 she conducted an intensive study of extremists' use of social media networks, but she says since then communications methods have dramatically changed.
"Terrorist recruiters are barely present on Twitter and Facebook anymore," she told AAP.
She said proposals by governments to hold big social media companies responsible were "two to three years out of date".
"The big apps all chose to scrub their sites clean and are doing a pretty good job," she said.
Encrypted communication apps are now the weapons of choice for terrorist groups such as Islamic State.
"Telegram is the app of choice. Even the Islamic State's official news agency uses it," Prof Klausen said.
"It is an encryption app and even the provider does not know the identity of users and account holders."
Telegram was invented by two Russians who wanted a system their own country's security services could not crack.
Based in Berlin, users are protected by the European Union's strict privacy rules created in the aftermath of the Edward Snowden revelations about the US National Security Agency.
Prime Minister Malcolm Turnbull's cyber security adviser Alastair MacGibbon said the government was well aware of the problems caused by encrypted communications apps.
Mr MacGibbon said law enforcement agencies may not need access to the messages themselves to chase extremists.
"Sometimes ... it's about metadata, other times it's going to be other investigative techniques used by law enforcement," he told ABC radio on Wednesday.
"It may be that just knowing that you and I are part of a network is enough.
"It might be that just you and I (are) communicating, when we communicated, and who else we communicated with is enough to further an investigation."