Marketing behemoth WPP has snapped up a non-controlling stake in Polish digital advertising agency Heureka Group.Heureka is a full-service interactive advertising agency that specialises in digital advertising and social media campaigns, with its own proprietary social analytics tool called Brand Fibres. As part of Chief Executive Sir Martin Sorrell's plans to keep WPP's digital agency advantage over its rivals, the group said it was looking to expand its services in fast-growing markets and strengthen its capabilities in digital media.The transaction will see WPP's digital arm VML purchase a 49% stake, with the option to acquire majority ownership in the future.The Polish company, which will rebrand as VML, last year turned over €6.8m but WPP did not reveal its profitability. WPP explained that it's group targets, including associates such as Heureka, were to generate more than $6.0bn digital revenue in 2013, representing over 33% of total sales. WPP added that it has set a target of 35-40% of revenue to be derived from digital in the next five years.WPP companies and associates in Central and Eastern Europe collectively employ around 6,000 people with revenues over $600m.Shares in WPP were up 0.6% at 1,108p at 09:55 on Thusday.OH