PITTSBURGH (AP) ― 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 rather an anti-spam computer algorithm that was impersonally 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,” Facebook spokesman Andrew Noyes 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, reported the problem, too. Some have even created new Facebook pages where people who’ve been blocked can commiserate.

“Our systems classify over 10 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 very active in protests over natural gas drilling. “Then I found out a few others were blocked, and we all started getting angry.”

Forouzan and others said this 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 also have their own Facebook pages, too.

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 only relate to hydraulic fracturing, or fracking, a process which 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.

Experts agree that Facebook and other social media sites must use automated programs to protect against spam, but said more can be done to protect innocent people from losing access. Erica Newland, a policy analyst at the nonprofit Center for Democracy & Technology in San Francisco, which promotes a free Internet, said that when Facebook notifies someone that they’ve been blocked from posting, it needs to provide a link for legitimate users to appeal.

Facebook has an appeals process for people whose personal accounts were disabled, but doesn’t appear to have one for the 15-day spam sentence.

That lack of an appeals process is one of the things that angered Burr Hubbell, a Pawling, N.Y., critic of gas drilling.

“I can’t figure out how to even get a response to an e-mail, let alone talk to a person,” said Hubbell, an attorney and stay-at-home-dad who was blocked about a week ago.

Hubbell said that during last year’s Gulf oil spill he posted a lot of comments on Facebook pages, and at one point he got warned about posting comments too quickly. But with the recent block, he said, “This came without any warning at all.”

Late Thursday, Hubbell and others got this message from Facebook:

“Your account was mistakenly blocked from posting on Pages. We apologize for any inconvenience this has caused. We’ve lifted the block from your account, and you should now be able to post again.”

On Friday morning he tried to post again ― and got the same message saying he was blocked for posting spam.

Now, Hubbell wonders if Facebook has really solved the problem.

Newland, the San Francisco policy analyst, said Facebook and other sites face incredible challenges, given the volume of content they handle. And she noted that the 15-day posting ban was a type of warning, since the activists weren’t kicked off Facebook.

“They have created their own rules, and are trying to enforce these rules, and are trying to do so in a generally fair way. Facebook’s relationship to the content is very different from a newspaper,” she said.

Many other people have had similar problems with social media, she said.

“The decision that a company makes about when to remove content can have a real impact on discourse,” Newland said. “It certainly highlights the need for greater transparency from Facebook.”

Facebook’s problem isn’t new. Polonetsky said that during the most-active era of Internet spam, AOL’s automated programs were catching and discarding 1 billion pieces of suspect e-mail every day. He also noted that since Facebook is free, it won’t have huge customer-service call centers. And even if it did, the staff wouldn’t be able to make complicated decisions about whether someone had been improperly blocked.

Forouzan and others who were blocked made regular posts to the Facebook page Gasland, which was originally set up to promote the controversial documentary of the same name that sharply criticized natural gas drilling practices around the country. The film, nominated for an Academy Award, has been equally criticized by the industry and some independent commentators for exaggerating the risks and negative impacts of drilling. The page has grown into a kind of national bulletin board on the issue of fracking, and now has 58,921 members.

Josh Fox, the director of Gasland, said Facebook has been a tremendous resource, and he understands the challenge it faces. He also wants the activists to get their posting rights back.