In its latest effort to catch up to Amazon in e-commerce, Walmart has agreed to buy "key assets" of Boulder, Colo.-based mobile ad targeter OneRiot for an undisclosed sum.

The deal comes five months after Walmart bought Twitter app developer Kosmix and repurposed it as @WalmartLabs, the giant retailer's Mountainview, Calif.-based R&D lab for e-commerce and social media.

So why does Earth's largest retailer, with some pretty big existing relationships with digital (see: R/GA) and media agencies (see: Starcom MediaVest Group) need an ad targeter?

Anand Rajaraman, Kosmix co-founder and and senior-VP e-commerce at Walmart, said on the @WalmartLabs blog in a post late yesterday that "OneRiot has developed some pretty nifty technology that analyzes social-media signals from popular networks like Twitter and Facebook to deliver ads that are relevant to consumers' interests."

It's an idea not unlike the Shopycat gift-recommendation engine @WalmartLabs has been testing, only for serving up ads on mobile devices, not gift ideas. As Mr. Rajaraman put it: "The technology at the core of what we do [at @Walmartlabs] is the Social Genome, which enables us to connect millions of consumers with the best products based on their interests at any given moment," he wrote. "The OneRiot technology will enrich the Social Genome, and the OneRiot team adds to the already deep expertise we have around social data analysis."