Wikipedia — that crowd-sourced, online repository of knowledge — announced plans to black out its content Jan. 18 in protest over the Stop Online Piracy Act and the Protect IP Act, two online piracy bills that the organization is worried will hamper free expression and the open Web.

The decision was based on a vote by 1,800 Wikipedia users, but for a site that Google AdWords registers as getting over 400 million visitors per month, that’s just a small fraction of the people that use the service every day.

That means that a lot of people are going to be very confused when they turn to search engines Wednesday morning to do some light research into the random facts of the day.

Pop just about any query into your preferred search engine and it’s likely that if there is a Wikipedia page on the subject, it will be one of your first hits. Can’t remember the capital of Vermont? Montpelier, third hit. Who was the 23rd U.S. President? First hit, Benjamin Harrison. Even a Google search for SOPA itself pulls up the Wikipedia entry as the first hit, after news.

Nearly half of all Wikipedia users end up there because of the site’s prime real estate at the top of search results, according to an October study from the site, and many people specifically look for Wikipedia results in search.

Another popular Web service, Reddit, is also blacking out its Web sites, as are other Web companies and organizations such as Boing Boing, Mozilla and the Cheezburger Network.

So how do sites like Reddit or Wikipedia pull their content offline?

For starters, they set up a page that tells consumers why they’re blocking their content and what they want their users to do to support their cause. In Reddit’s case, the company will have a homepage that provides users with information about why the service is down and gives them the opportunity to add their voice to the protest.

A service as big as Reddit “doesn’t take this decision lightly,” said the social news service’s co-founder, Alexis Ohanian. “We want [the service] to be down for the day, but don’t want to be totally crippled the next day.” Reddit is telling search engines such as Google that, essentially, its site is temporarily down and will be directing users to a main page to keep them updated on its protest activities around the country.

Wikipedia is taking a different approach, pulled from their fundraising playbook, that will place a graphic outlining Wikipedia’s position on every page — likely a dark screen, with a “W” and links pointing to ways users can take action, according to Wikimedia’s head of communications, Jay Walsh.

Google declined to comment officially on how it will deal with the blacked-out site in its search. A thread on Google’s Webmaster forums , however, recommends that sites that want to participate in a blackout use a certain type of code that essentially tells the search engine that the site is temporarily down. That way, the sites will be blocked from the public Web without being identified as a broken site.

Google said it will support the anti-SOPA cause through a link on its homepage rather than a blackout. Microsoft, which runs the Bing search engine also said that it opposes SOPA, and will not black out its search results.

“[Hundreds] of millions of customers rely on our services every day, so we don’t plan to shut those down to express our view,” a Microsoft spokesperson said in an e-mailed statement. “This is an important issue and we think the recent White House statement points in a constructive way to problems with the current legislation, the need to fix them, and the opportunity for people on all sides to talk together about a better path forward.”

Supporters of SOPA have been quick to call these protests a “disservice” to these Web site daily users.

“It is ironic that a Web site dedicated to providing information is spreading misinformation about the Stop Online Piracy Act,” said SOPA sponsor and chairman of the House Judiciary committee Rep. Lamar Smith (R-Tex.) “The bill will not harm Wikipedia, domestic blogs or social networking sites. This publicity stunt does a disservice to its users by promoting fear instead of facts. Perhaps during the blackout, Internet users can look elsewhere for an accurate definition of online piracy,” he quipped.

Christopher Dodd, the former senator and current head of the Motion Picture Association of America, went further, saying that blacking out these sites is an “abuse of power given the freedoms these companies enjoy in the marketplace today.” He said that these protests are dangerous, setting a precedent for “platforms that serve as gateways to information to intentionally skew the facts.”

In response to that criticism, Walsh said that Wikipedia wouldn’t consider an action like this “if we weren’t facing this threat. Whatever anyone else thinks, these bills are a clear threat to the free and open Web.”

Wikipedia founder Jimmy Wales said he would like to see Congress make a fresh start on online piracy legislation.

“I think it would be better to burn them down and start over,” he said. “The entire approach is philosophically wrongheaded. I think you should follow the money ... don’t place the burden on the innocent third-parties on the Internet,” he said. “Censorship is never going to be the right answer.”

Related stories:

Google to state anti-SOPA stance on home page

SOPA: Twitter will not join Wikipedia, Reddit in blackout

Wonkblog: Sen. Ron Wyden’s fight to save the Internet