This year, Digg entered a decline after a series of stumbles, and some users were said to defect to rival Reddit. And yet the site's troubles really began much earlier, with the rise of Twitter and Facebook. Although radically different in their execution, these two social services served much the same purpose as Digg by allowing users to share interesting links they found around the web. As web users found new outlets for these behaviors, Digg's relevance dwindled.
The real threat to Digg wasn't a clone but rather a completely different approach.
Google is now feeling the heat from Facebook. Although hundreds of "Google killers" have come and gone, all of them trying to tackle Google head-on, it looks like Facebook might be the most credible threat so far to Google's core search business.
Incremental improvements almost never create new market leaders. Instead, it takes a completely different approach to unseat incumbents.
If a true "Facebook killer" should ever emerge, it probably won't resemble Facebook at all.
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