My sense is that people just assumed Pareto Principle numbers (80/20 rule) and are making an implicit comparison to that. And they assume participation = conten

My sense is that people just assumed Pareto Principle numbers (80/20 rule) and are making an implicit comparison to that. And they assume participation = content-creation = uploading photos. But I agree with you. Uploading photos is only one way to participate on Flickr, it's not the only way. I go to Flickr to upload my photos, but I also go just to look at my photos (which counts as a visit), and I encourage my friends to do so as well. That's what photos are for after all. I also comment on the photos of others and sometimes "favorite" them. I think the implicit definition of participation in the article is too narrowly defined, at least in the case of Flickr.