Disintegrating Digg
Fragments, factions, and turf warfare may force Digg to accommodate greater personalization of content. This could nullify the importance of voting stories onto the homepage, which is largely the reason for Digg's success.
A great contradiction and Catch-22 for the site's architects, and a dilemma that is not completely unrelated to agency-vs-structure disputes in social theory. However, I wouldn't say with absolute certainty that Digg demonstrates the failure of open collaborative networks because I think we are still in the process of discovering and inventing what an open collaborative network actually is.