Little Black Book (www.lbbonline.com) founder Matt Cooper launched the business in 2009 as a specialised resources directory for creative companies and professionals in New York and London.
The online portal has since grown into a community of more than 40,000 advertising and media professionals, hailing from more than 40 cities around the world.
LBB features information about the local media landscape, while offering a social networking platform with each city-guide created and recommended by the creative community itself.
Gerard Barron, chief executive of Adstream, said: "The impressive success of Little Black Book in connecting the advertising community at a personal level seemed to us an ideal complement to Adstream's own core capabilities of helping business to connect and interact more efficiently, both locally and internationally.
"We are extremely proud to welcome Matt Cooper to our team and have great plans to take the success of Little Black Book even further."
Cooper, who joins Adstream together with the rest of the LBB team, will be based in Adstream's London offices and will be responsible for the continued growth of the LBB online portal, in addition to supporting the development and commercialisation of Adstream's digital asset management platform AdBank.
He said: "I am thrilled in having Little Black Book as part of Adstream. They are recognised not only by their powerful technology but also by their fantastic team - they understand it's a people business and we can see immediate and long-term benefits to Little Black Book clients and end-users. By working with Adstream we can now take Little Black Book to the next stage."