NEW YORK (AdAge.com) -- One of the ways that Omnicom's BBDO is adjusting to the digital age is by growing its Proximity network of ad agencies. Created about six years ago, the heavily digital Proximity has 2,000 employees and offices in 55 countries. It's now expanding into the U.S. with a presence in New York, Chicago, Minneapolis and Cincinnati. BBDO CEO Andrew Robertson details the recent success of its chocolate mail and tree-house campaigns.