Twitter spokesperson Carolyn Penner just published a post on the microblogging service's blog that more fully outlines how Trending Topics are determined on Twitter.
In essence: Twitter favors novelty over popularity. Trending Topics are "designed to help people discover the 'most breaking' breaking news across the world… Captur[ing] the hottest emerging topics, not just what's most popular," Penner writes.
"Topics break into the Trends list when the volume of Tweets about that topic at a given moment dramatically increases," she adds.
From this explanation, we can infer that the reason the WikiLeaks hasn't trended this week, despite being the most-discussed topic on Twitter at times, is because there hasn't been a dramatic increase in the level of discussion about WikiLeaks compared to previously.
It's for that very reason that other popular topics -- like, ahem, Justin Bieber -- have not trended as of late, Penner says.
Yet according to data pulled from Trendistic (via blogger Bubbloy), WikiLeaks discussion has spiked significantly as of late, and yet #WikiLeaks has not trended since August 26:
