Web2Con: Popularity Decay in Tagging

The following bit emerged out of the What’s in a Tag session at the Web 2.0 Conference.

One of the great features of tagging is seen when tags are aggregated. Then, we can see trends in what people are tagging: valuable bookmarks on Del.icio.us, cool pictures on Flickr, for example. These trends are trends of popularity: what many people are doing.

One of the major problems with popularity is that popularity breeds popularity. When something gets popular enough, it is introduced to newer, wider audiences. These audiences, unaccustomed to the new idea, keep it popular. Unfortunately, these cycles tend to be too long: new things that become popular on a wide scale drown out those newer ideas that are still on the small scale, waiting their turn at the big time. These smaller ideas might be just as valuable as the popular ideas, they just haven’t had their chance to shine. Those folks who already know about the popular idea and are waiting for the new idea are out of luck.

The rate at which something loses its popularity is called popularity decay. It is similar to radioactive decay, except that radioactive decay is difficult to control. Popularity decay, on the other hand, is controllable to a large extent.

A great example of designers manipulating popularity decay is Flickr’s Popular Tags page, as shown during the session by Caterina Fake of Flickr/Yahoo. Instead of showing a generic “what’s popular” list of the most popular tags, this page shows three stages of popularity, with three different levels of popularity decay.

One stage contains those tags that were popular in the last 24 hours. This stage has a 24 hour popularity decay. Things that are popular today might not be popular tomorrow.

A second stage contains those tags that were popular in the last week, and thus a week-long popularity decay. Things that are popular this week might not be popular next week.

The third stage is all-time popularity, the default popularity for most systems, and it clearly shows the effect of a slow popularity decay. The popularity decay of this stage is never reset, as it is in the other stages. For something to be added to this list, it has to actually be more popular over all-time than the other things on the list. Therefore, turnover is very slow, and thus watching this list won’t tell you very much about trends as they happen. When some idea hits this stage, you can be sure that it is popular, but it certainly isn’t new, because it has had to be popular over the very long term.

The idea of popularity decay illustrates the tension between new ideas and popular ideas. Both are valuable, but not at the expense of the other.