Why didn’t #FreddieGray trend?

More than 1,000 people descended on Baltimore last weekend to protest against the death of 25-year-old Freddie Gray. Gray was a black man who died in police custody — a death now ruled a homicide — and whose name has been listed alongside those of Michael Brown and Walter Scott as evidence of mounting police brutality. The Baltimore protests got violent. Six police cars were damaged and police in riot gear arrested 12 men.

The story got prominent coverage on CNN and ABC, and in papers from the New York Times to USA Today. More than 150,000 people tweeted April 25 on the #FreddieGray hashtag. And yet, amidst all that furor, #FreddieGray never trended nationally on Twitter. According to Trendinalia, a site that tracks historical Twitter trends, it never got above the No. 21 position.

It means the protest didn’t surface in the right-hand rails and newly prominent trends tabs of millions of Twitter users, who may not otherwise have heard the news.

As is often the case with this type of thing, an algorithm is to blame.

As the data scientist Gilad Lotan explained in an analysis published Friday, the algorithmic system that determines Twitter’s “trending” topics favours things that spike quickly, not topics that build a large, sustained audience over time.

That makes sense, when you think about it: if trending topics were determined by volume alone, we’d never get anything that didn’t involve Zayn Malik. (And it often feels like that, anyway.) But even though the system makes sense, it often produces results that don’t.

Lotan initially made this discovery about the Twitter algorithm in 2011, when the hashtag #OccupyWallStreet failed to ever trend in its home base, New York. Meanwhile, momentary drivel such as #KimKWedding and #ICantRespectYouIf did surface.

“If we see a systematic rise in volume, but no clear spike, it is possible that the topic will never trend,” Lotan wrote at the time. That means three things, he said:

1. The longer a term stays in the trending topic list, the higher the velocity (of tweets) required to keep it there.

2. It is much easier for a term never seen before to become a Twitter trend.

3. It is extremely important to understand what else is happening in the region or network. (If Kim Kardashian’s show is airing, you can forget about trending.)