Menlo Park, Calif.--Score another well-trafficked Web site for DoubleClick. On Monday, eFax.com said it has signed with DoubleClick's fledgling Sonar Network to deliver advertising to its 1.7 million business-to-business subscribers. EFax.com, Menlo Park, Calif., has modified its Web site technology to support DoubleClick's reporting and targeting software--called DART--and will begin to deliver advertisements to both the sender and receiver of messages. The move is a big boost for DoubleClick, New York, which announced the formation of Sonar Network late last month. The new service is aimed at direct marketers, seeking lower-cost advertising CPMs on highly targeted sites. Since eFax.com is a major business portal, its entry on Sonar Network paves the way for business-to-business advertisers to use the service.