Facebook has sent notes of apology and is changing automated systems that blocked environmental activists and other people from posting on like-minded Facebook pages.
The activists weren't victims of censorship, but of an anti-spam computer algorithm that impersonally was doing what it was designed to do.
"Facebook is not -- and has never been -- in the business of disabling accounts or removing content simply because people are discussing controversial topics," Andrew Noyes, Facebook spokesman, said in a statement to the Associated Press. "On the contrary, we want Facebook to be a place where people can openly express their views and opinions, even if others don't agree with them."
The activists were flagged by an anti-spam program and told they couldn't post for 15 days. Other users, including an animal rescue activist, also reported the problem. Some even have created new Facebook pages where people who have been blocked can commiserate.
"Our systems classify over10 billion actions (suspicious logins, friend requests, etc.) and pieces of content (messages, Wall posts, etc.) every day," Noyes said. "Of course, no system is perfect, and we do sometimes make mistakes."
The activists weren't blocked by a page administrator for making off-topic posts or for offering questionable commercial services. They couldn't even post to pages run by people who agree with their views.
"The first feeling was surprise, because I'd been doing this for over a year, with no problem," said Gloria Forouzan of Pittsburgh, who has been active in protesting natural gas drilling. "Then I found out a few others were blocked, and we all started getting angry."
Forouzan and others said last week they still don't know what they did to trigger the blocks.
Their reaction also shows just how important Facebook has become to a wide range of groups who use the free service to network and spread messages. Pro-gas industry groups have their own Facebook pages.
Facebook didn't provide details of the problem. To do so, they said, might help spammers find ways around the anti-spam software.
Others note that people would complain if Facebook weakens its anti-spam programs too much, since spam would surge.
"Navigating that exact balance is always quite tricky. It's automated, done by algorithm, blind to the political value of the message," said Jules Polonetsky, the former chief privacy officer for AOL and now a director of the Future of Privacy Forum, a Washington, D.C., think tank.
Forouzan said she posts links and comments to Facebook pages critical of gas drilling "every day, several times a day."
She said the posts relate only to hydraulic fracturing, or "fracking," a process that injects chemical-laced water deep underground to break up shale rock and allow natural gas to escape.
"Never jokes, never pat-the-bunny stuff," she said.