Staff at the whistleblowing website WikiLeaks were preparing a legal challenge last night after discovering that US officials had issued a subpoena on their Twitter account to demand private messages, billing addresses, and other details. The group suspects that many other social networking sites and email providers will also have been asked to reveal their private conversations.

The site's founder, Julian Assange, was one of four figures associated with the group to have a subpoena on their account. He said: "If the Iranian government was to attempt to coercively obtain this information from journalists and activists of foreign nations, human rights groups around the world would speak out."

The revelation suggests that an investigation ahead of a possible US grand jury hearing is likely to have begun trawling for details of any WikiLeaks communications with Bradley Manning, the US army intelligence analyst suspected of handing classified information to the site. Also targeted were a high-profile Icelandic MP Birgitta Jonsdottir, Dutch hacker Rop Gonggrijp and US programmer Jacob Appelbaum, all of whom had previously collaborated with WikiLeaks.

A court order issued on 14 December ordered Twitter not to disclose the existence of the subpeona to any of those targeted because it was an ongoing criminal investigation. Only after a legal action from Twitter was the order made public. WikiLeaks spokesman Kristinn Hrafnsson said: "We are now assuming that all social media have had similar orders: Facebook, Google, everyone. We don't have confirmation of that, but we must assume it because this one was made in secret. We are meeting with our lawyers about the legality of these subpoenas."

Like Assange, Icelandic politician Jonsdottir has said she will fight the order. In a message posted on Twitter she said she had "no intention to hand my information over willingly". Dutch hacker Gonggrijp said: "It appears that Twitter, as a matter of policy, does the right thing in wanting to inform their users when one of these comes in. Heaven knows how many places have received similar subpoenas and just quietly submitted all they had on me." Twitter has declined to comment on the claim, saying only that its policy is to notify its users, where possible, of government requests for information.

The American government has been furious with WikiLeaks ever since they published classified military documents on the wars in Iraq and Afghanistan. The site's recent release of the first of tens of thousands of US embassy cables has not made them any more popular with US officials, who are now looking at what charges they can bring.

Google's London office did not immediately return a call and an email seeking comment. Facebook did not immediately return an email seeking comment either.

Although its relations with the US government have been ugly, WikiLeaks and its tech-savvy staff have relied on American Internet and finance companies to raise funds, disseminate material and get their message out. WikiLeaks' frequently updated Facebook page, for example, counts 1.5 million fans and its Twitter account has a following of more than 600,000. Until recently, the group raised donations via PayPal Inc, MasterCard Inc, and Visa Inc, and hosted material on Amazon.com's servers, but the group's use of American companies has come under increasing pressure as it continues to reveal US secrets, with PayPal and the credit card companies severing their links with site. Amazon.com booted WikiLeaks from its servers last month.

Assange will appear in a London court this week for the next stage of hearings into Sweden's extradition request. Prosecutors there want to interview him about alleged sex offences.