Pressure was growing for Facebook and Google to state whether they have been asked to submit data on prominent figures linked to WikiLeaks, as fresh signs emerged that a secret jury investigation will lead to an attempt to lay criminal charges against the whistleblowing website's founder.
Julian Assange and four others are the subjects of an American court order requiring the surrender of details about their Twitter accounts. The court order states the information could be relevant to an "ongoing criminal investigation".
The US is attempting to build a criminal case against Mr Assange by accusing him of conspiring with Bradley Manning, the US military intelligence worker suspected of providing WikiLeaks with more than 250,000 secret diplomatic cables.
Mr Assange is in Britain awaiting the results of extradition proceedings to Sweden on sexual assault charges.