Until Aug 2009 , Shutterstock used fairly straightforward algorithm for “Most Popular search” – it took into account number of downloads and the length of time the image was on sale, e.g.  photo with 2 downloads in 4 days would have the same position as photo with 4 downloads in 8 days. In Aug 2009, Shutterstock announced changes to their “Most Popular”. Here is official info from Shutterstock:

For each image in our library, we calculate a popularity score based on the total number of downloads, the length of time the image has been available to customers on our site, and a number of other proprietary metrics. Organizing your search results by Most Popular images first, currently the default setting, ensures that you’ll see the images we have identified as the highest-quality images most relevant to your search.

Images ranked high in “Most Popular” have better sales (and better sales would boost their standing in “Most Popular”) and longer shelf-life. Getting them there is a science, keeping them there is an art.

Most microstocks have their own versions of “Most Popular”.  See relevant wiki artciles:

Istockphoto – “Best Match”

Fotolia – “Relevance”

Dreamstime – “Relevancy”

Stockxpert – “Popularity”

Related posts:

1. Best Match – Istockphoto (wiki)
2. Popularity – Stockxpert (wiki)
3. Relevance – Fotolia (wiki)