Yahoo has expanded its “Buzz Up” content recommendation service to include many more publishers, stepping up its challenge against Digg. Almost any publisher can now add a Buzz Up button to its Web site, Yahoo said. If a Web site doesn’t have a button, users can also submit a link to the content via Yahoo.

Since its launch in February, Yahoo slowly built up the number of publishers participating in the service from 100 to 400. It has now opened it to almost any publisher — although it said the service is “currently focussed on the U.S. only.”

The change means that users will be able to “buzz up” more types of content, including the news, video and blogging sites that helped drive the popularity of Digg and similar sites such as Reddit and StumbleUpon.

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