The background: the Tinder Aids project is an initiative that came up at a time when Tinder became a very popular app used by millions every day.
The idea: use Tinder to spread a positive social message regarding safe sex, as the app is known to be used primarily to find casual sex partners.
The solution: creating fictitious Tinder users and photoshop their photos with multiple partners inside the app, and in the last picture, putting the following message: “You’re probably not her only match. Use a condom”, accompanied by a red AIDS ribbon. 
The results: Thousands of users were exposed to the message, and lots of great user comments were received. All of this, with zero budget.