Facebook is facing down another embarrassing episode of censorship this week after refusing to show ads submitted by the Just Say Now marijuana legalization campaign. The gag is an important reminder that social networks like Facebook — while useful, interesting, and pretty — are "walled gardens" with overseers whose interests can overwrite free speech, open communication, and in this case, essential political debate. (In this they have something in common with Apple.)

Most recently, Facebook was caught censoring mentions of Power.com, an online tool designed to help users collect their information from Facebook to facilitate migration to other social networks. To this day, users are still blocked from sending messages or posting status updates containing the word "Power.com," preventing users from spreading the word about a convenient way to "make the move" to Orkut, or LinkedIn, or any other social networking service that may crop up to compete. The block even stopped law professor Eric Goldman from commenting on Facebook’s lawsuit against Power.com (Disclosure: EFF filed an amicus brief in support of Power in that case).

Facebook's censorship for anticompetitive reasons is petty and lame to be sure, but silencing Just Say Now's marijuana legalization ad campaign is even worse. Voters in various districts nationwide will have to make important political decisions about marijuana this year (California's Proposition 19 is one example). Facebook's decision, reportedly an attempt to be consistent with its ad policies restricting smoking and/or marijuana-related content, is instead primarily silencing an important, motivated voice in a politically significant debate.

Facebook should lift the ban and show Just Say Now's political ads. For better or worse, Facebook has become a important means of communication and organization for candidates and political campaigns. In this role, Facebook functions best as a neutral platform, hosting the debate without entering it. Whether or not Facebook wants to restrict depictions of smoking in commercial ads, it should not prohibit the open and robust political debate central to the value and promise of the Internet.