Pyr0 (aka Valentin Laube) did a nice visual comparison between two different patterns for social software. On the one hand the service oriented approach where users are scattered around separate communities and on the other hand the rather decentralized approach where users can control the way they interact with other users on the basis of different types of shared resources.
The user icons are taken from the wonderful Silk icon set by FamFamFam.
Apart from this chart Valentin is involved with Atomique in different ways. We both implemented a prototypical functionality allowing a simple type of content-based image retrieval. Furthermore did he create a niftier favicon, which you might see in your address bar. Thanks 
The greatest challenge in the decentralized approach is to connect the users. Trackbacks in blogs and Atomique’s groups only solve a part of this, they only connect the users that stumble across the blog entry with the trackback or the group on some Atomique page. When the user wants to search for information, a centralized approach is needed again. Specialized search engines like Technorati for blogs or the Google Image Search are already doing a good job to close that gap. But right now search engines for images are indexing new pages way too slow to be of much use for something like Atomique where users are constantly uploading new photos. Maybe this will change in the future, else we have to find a way to do that ourselves
Yes, for search and discovery there still is a need for some kind of centralized search engines (note the plural here). I think Technorati’s approach in the blogosphere is a viable way of finding a compromise between completeness and speed. Blog entries are added by both a crawler and a ping service. This can be done for other types of media beyond blog posts such as photos, videos, audio, as well. Personal repositories would ping those “trackers” the users like the most, and the remaining trackers would still find those resources - later though. The pinged trackers would have the newly added entry immediately.