Matt Marshall is leaving his job as a business reporter at the Mercury News on Friday to run VentureBeat, a new Web site that is a successor to the SiliconBeat site he started with fellow Merc writer Michael Bazeley nearly two years ago. The Merc plans to syndicate VentureBeat on its Web site, Marshall said in an introduction to his new site. The site will include hard news about the Silicon Valley start-up world, discussion of the news as it unfolds, and opinion pieces from contributors.