A hacking free-for-all has exploded on the Web, and Facebook and Twitter are stuck in the middle.
On Wednesday, anonymous hackers took aim at companies perceived to have harmed WikiLeaks after its release of a flood of confidential diplomatic documents. MasterCard, Visa and PayPal, which had cut off people's ability to donate money to WikiLeaks, were hit by attacks that tried to block access to the companies' Web sites and services.
To organize their efforts, the hackers have turned to sites like Facebook and Twitter. That has drawn these Web giants into the fray and created a precarious situation for them.
Both Facebook and Twitter -- but particularly Twitter -- have received praise in recent years as outlets for free speech. Governments trying to control the flow of information have found it difficult to block people from voicing their concerns or setting up meetings through the sites.
At the same time, both Facebook and Twitter have corporate aspirations that hinge on their ability to serve as ad platforms for other companies. This leaves them with tough public relations and business decisions around how they should handle situations as politically charged as the WikiLeaks developments.
Some internet experts say the situation highlights the complexities of free speech issues on the Internet, as grassroots Web companies evolve and take central control over what their users can make public. Clay Shirky, who studies the Internet and teaches at New York University, said that although the Web is the new public sphere, it is actually "a corporate sphere that tolerates public speech."
Marcia Hofmann, a lawyer at the Electronic Frontier Foundation, said, "Any Internet user who cares about free speech or has a controversial or unpopular message should be concerned about the fact that intermediaries might not let them express it."
She added, "Your free speech rights are only as strong as the weakest intermediary."
The problem came into relief on Wednesday, through an effort called Operation Payback, organized by a group calling itself Anonymous. The group spent much of the day posting notes on Facebook and Twitter that told followers which companies to single out and that documented hacking successes.
But Facebook banned one of the group's pages, saying that using the site to organize hacking attacks like that violated its terms of use. The group went on Twitter to complain.
A Facebook spokesman issued a statement saying that the company was "sensitive to content that includes pornography, bullying, hate speech, and threats of violence" and would "take action on content that we find or that's reported to us that promotes unlawful activity."
In an interview Wednesday morning, Joe Sullivan, Facebook's chief security officer, addressed WikiLeaks's own presence on the site. He said the company had not received any official requests to disable pages or accounts associated with the WikiLeaks organization.
Facebook generally resists requests by governments or advocacy groups to take down material if that content is not illegal or does not violate Facebook's terms of service, which prohibit attacks on individuals or incitements to violence.
"Facebook is a place where people come to talk about all sorts of things, including controversial topics," Mr. Sullivan said. It was not clear whether anyone had asked Facebook to take down the Operation Payback page.
Twitter allowed the Operation Payback account to stay active most of Wednesday. But the group's account was disabled late in the day, after it posted a link to a file that provided thousands of consumer credit card numbers, according to a person with direct knowledge of the situation.
A Twitter spokesman declined to discuss the details of the situation. "We don't comment about the specific actions we take around user accounts," he said.
The company is not overly concerned about hackers' attacking Twitter's site, he said, explaining that it faces security issues all the time and has technology to deal with the situation.
Twitter is in a particularly delicate situation because its founders have celebrated their service's role in political protest and free speech. They have not been shy about trying to capitalize on the good will engendered by playing that role.
WikiLeaks's own Twitter account remains active, and it is the group's main channel for reaching supporters and the media.
Last week, Amazon.com fell into a similar position when it decided to stop storing files for WikiLeaks. Advocates of WikiLeaks complained that Amazon.com was bowing to political pressure to cut the organization from its Web services. An Amazon.com spokesman said the company was simply banning an organization that had violated its terms of service by trying to distribute documents it did not own.
The last week has given rise to a hacking war in which groups have blocked access to WikiLeaks's Web sites by bombarding them with requests.
And now the WikiLeaks supporters have responded in kind, flying the freedom of speech banner as the motivation for their actions.