On Friday, Facebook will introduce a tool that will enable online retailers to track purchases made by the social network’s users who saw their ads. This is a play to convince marketers that spending money on Facebook advertising actually pays off. The company is calling this a “conversion measurement” tool and notes that this has been a top customer request for a long time. Marketers can use the tool to generate a bit of code to add to any page on their website where they want to measure conversions. It will be available on Facebook’s self-serve ad platform. Facebook tried to launch a conversion-tracking tool back in 2010, but it ended the beta test after less than a year. The difference this time around, according to the company, is the combination of conversion measurement and Facebook’s Optimized CPM bidding option, which allows advertisers to set marketing goals and have the social network’s system automatically serve ads to users who are most likely to convert. (Reuters, Inside Facebook)