was the first affiliate network to launch an offering for users of social networking sites, working with the travel-focused social network WAYN.
It was established in the UK in 2002 and now has offices in London, Newcastle and New York, employing a total of 70 people.
Randy Falco, AOL chairman and chief executive, said: "We look forward to accelerating the expansion of buy.at's affiliate network in the United States and worldwide."
Time Warner-owned AOL's new acquisition will operate as a business unit of Advertising.com, which is part of AOL's wider advertising services offering Platform A.
Advertising.com operates in the US, Japan and nine European countries.
Buy.at is the fifth advertising acquisition AOL has made in the past 12 months. These include contextual ads firm Quigo, mobile ad network Third Screen Media, online ad serving company AdTech, and video content-linked ads company Lightningcast.