Is Twitter censoring Occupy Wall Street?

In an era of political upheaval and economic turmoil why does Twitter tell us that "Bieber" is a trending topic? Twitter execs say it is all about the algorithm.
Organizers of political movements that use the private social network to spread their message and grow their cause are increasingly suspicious that their "hashtags" are being left out of Twitter trends.
Trending topics is one way Twitter informs users what is being discussed right now on the social network. But the widely used hashtags #ows and #occupywallstreet used in many Tweets about Occupy Wall Street have rarely pushed the topic to the coveted Trending list. The same was true for #wikileaks when the organization released oodles of confidential and diplomatic cables not intended for public consumption.
Tarleton Gillespie of Microsoft Research writes on the Social Media Collective blog:

It (Twitter) engages in traditional censorship: for example, a Twitter engineer acknowledges here that Trends excludes profanity, something that's obvious from the relatively circuitous path that prurient attempts to push dirty words onto the Trends list must take. Twitter will remove tweets that constitute specific threats of violence, copyright or trademark violations, impersonation of others, revelations of others' private information, or spam. (Twitter has even been criticized for not removing some terms from Trends, as in this user's complaint that #reasonstobeatyourgirlfriend was permitted to appear.)

But censorship, the company says, isn't what's happening with Occupy Wall Street.
Gillespie explains:

Wikileaks may not have trended when people expected it to because it had before; because the discussion of #wikileaks grew too slowly and consistently over time to have spiked enough to draw the algorithm's attention; because the bulk of messages were retweets; or because the users tweeting about Wikileaks were already densely interconnected. When Twitter changed their algorithm significantly in May 2010 (though, undoubtedly, it has been tweaked in less noticeable ways before and after), they announced the change in their blog, explained why it was made - and even apologized directly to Justin Bieber, whose position in the Trends list would be diminished by the change. In response to charges of censorship, they have explained why they believe Trends should privilege terms that spike, terms that exceed single clusters of interconnected users, new content over retweets, new terms over already trending ones. Critics gather anecdotal evidence and conduct thorough statistical analysis, using available online tools that track the raw popularity of words in a vastly more exhaustive and catholic way than Twitter does, or at least is willing to make available to its users. The algorithms that define what is "trending" or what is "hot" or what is "most popular" are not simple measures, they are carefully designed to capture something the site providers want to capture, and to weed out the inevitable "mistakes" a simple calculation would make.