The US Senate Commerce Committee has voted to subpoena the heads of Google, Facebook, and Twitter.

It means that Sundar Pichai, Mark Zuckerberg and Jack Dorsey face being quizzed by senators about their policies on different types of content.

One focus will be the legal protections they enjoy regarding what they leave up and take down on their platforms.

They are also likely to be challenged over controversies about privacy and misinformation.

Republicans and Democrats on the committee voted unanimously to summon the chief executives.

However, each side is likely to have different priorities - with some Republican members speaking about the alleged censorship of conservative views online, while Democrats focus on competition and misinformation.

"There has never been such an aggregation of power in the history of humankind as big tech enjoys today, with money and monopoly, power and the hubris that comes with the unchecked use of power," Republican Senator Ted Cruz said following the vote.

The subpoenas follow an earlier hearing in July when Amazon, Apple, Facebook and Google's chiefs testified to a House committee about competition issues and claims of bias.
Protected status

Currently, social media platforms and other websites are protected from prosecution under Section 230 of the Communications Decency Act.

It means, in effect, that the platforms are not liable for what is posted by their users if they take things down once notified, unlike traditional newspaper publishers.

But the protection has been thrown back into the spotlight this year after President Trump argued it should no longer apply if social networks were engaged in "an editorial role" - such as when Twitter began adding warnings and labels to some of the president's tweets.

Democrats on the Commerce Committee had initially objected to their Republican colleagues' use of the subpoena power, but changed their minds for the vote on Thursday after the scope of the discussion was widened.

Mr Trump's Democratic rival in the forthcoming election, Joe Biden, has previously said Section 230 should be revoked, but for different reasons - what he called "propagating falsehoods".

The three chief executives had initially been invited to attend voluntarily, but declined.
Fresh scrutiny

The fresh summons is the latest regulatory headache for the tech giants, who face a raft of critical decisions by authorities in the coming weeks.

July's congressional hearing with the companies, plus Amazon, dealt with anti-trust issues, including allegations that they had deliberately eliminated smaller rivals in a quest for market dominance.

The congressional report into that event is expected to be published next week.

The US Department of Justice is understood to be in discussions with state attorneys general over a potential anti-trust case against Google over its practices - a legal action which could also begin as early as next week.

And similar actions are being pieced together abroad.

The European Union's draft version of a forthcoming law reportedly bans tech giants from giving their own products preferential treatment.

The draft of the Digital Services Act, seen by a number of news outlets, says that big tech "gatekeepers" may have to share user data with rivals to create an even playing field in some circumstances.

And in China, Reuters news agency reports that the country is preparing its own competition probe into Google over the Android phone operating system.

US sanctions on Huawei on national security grounds have created a challenge for the company - one of the world's biggest phone manufacturers - which says it will begin using its own non-Android operating system next year.

That decision is also expected this month.