Today’s massive student protests in Britain have produced a huge amount of Twitter traffic, much of it using the #demo2010 hashtag. But although a number of tags and phrases connected with the demonstrations have trended in the UK over the course of the day, #demo2010 has been strangely — and entirely — absent from the country’s trending topics list.

Why that’s the case still isn’t clear, but a comparison of the relative frequency of tweets using that tag with phrases that have trended in the UK today reveals that it’s not because of lack of traffic.

(click on graph to see it full size)

As of right now — nearly 3 o’clock in the afteroon NYC time — the top four trending topics on UK Twitter are #uksnow, #dayx, #wonderlandwednesday, and McKeith. But as this chart shows, #demo2010 has more than double the traffic of any of them at this hour, and has been far ahead of them all for the entire day. At around noon London time today #demo2010 peaked at 0.2% of total Twitter traffic, according to this chart — more than triple the volume of any of the current top four.

So what’s going on? I’m going to continue to dig, and others are on the case as well. More soon.

Update | Via Twitter, in response to this post, comes the suggestion that #demo2010 can’t trend because there’s a Twitter user with @demo2010 as their handle, and Twitter doesn’t allow strings that are identical to usernames to trend.

Second Update | The above explanation is incorrect. There’s a @dayx account, has been since 2008, and #dayx has been trending all day.

Third Update | Twitter honcho Del Harvey (@delbius) says, in response to a tweeted query, “we aren’t blocking” #demo2010. In followup tweets she guesses that the tag’s failure to trend may be an artifact of Twitter’s algorithms — “The more something is talked about, the more it needs to be talked about in order to be a trend” — but offers no inside info as to what’s going on.

If the trending algorithms are keeping #demo2010 off the list — and if the stats I’ve seen for #demo2010 tweets are accurate — then I must say that the algorithms strike me as seriously flawed. Take a look at what happens when you expand the graph linked above to cover the last thirty days, instead of the last 24 hours…

What this shows us is that #demo2010 spiked high on November 10, and about half as high — but still really high — today. Both spikes were dramatic, and both reflected huge interest in the topic. More importantly, both spikes were spikes — today’s pop in the numbers reflects a rebound from something close to zero between November 13 and November 22.

Compare that to “McKeith,” which did trend today. That term had two huge spikes in the last couple of weeks, and fairly consistent traffic in between and after. It trended even though its traffic today was actually quite similar to the traffic its been seeing over the last ten days.

I’m not taking a conspiratorial view of this, but I do think it’s weird, and it does strike me as bad coding — if in fact coding is the problem.

Fourth Update | Tweeter @adrianshort argues that the “diversity of tweets for TV programmes” versus the “huge numbers of RTs” for #demo2010 may explain the latter’s failure to trend. (Gillian McKeith is currently appearing on “I’m a Celebrity … Get Me Out of Here!” in the UK.) And though there have been a lot of unique #demo2010 tweets today, there have also been a lot of RTs, so this explanation strikes me as the most plausible I’ve heard so far.

In a separate tweet, Short suggests that trending topics may not actually be such a huge deal for an event like today’s demonstrations, since “You get massive network effects from normal RTing a popular tag.” I’m inclined to agree with that, too.