Targeted advertising is a key characteristic of online as well as traditional-media marketing. However, it is very limited in outdoor advertising, that is, performing campaigns by means of billboards in public places. In this work we propose a methodology for performing targeted outdoor advertising by leveraging the use of social media. In particular, we use the Twitter social network to gather information about users' degree of interest in given advertising categories and about the routes that they follow. Given an advertising category, we estimate the most promising areas to be selected for the placement of an ad that can maximize its targeted effectiveness.