Costco is building out an ad network built on its trove of loyalty membership data, using its 74.5 million household members' shopping habits and past purchases to power targeted advertising on and off its website. The wholesale retailer is still testing such capabilities, and it's fielding offers from potential ad-tech vendors, but the move signals that the third-largest retailer in the US could become a formidable player in the already deeply fragmented retail media space, which is estimated to reach $166 billion by 2025, and responsible for 20% of all digital media spend this year.