Stefani co-founded Citizenside with Philippe Checinski and Julien Robert in 2006, raising funds through investments by the Agence France-Presse (AFP) and IAM in 2007.
The agency has since formed partnerships with a number of French media outlets including 20 Minutes, Metro, Le Parisien and Voici, which use Citizenside's Reporter Kit to collect images from users.
"It is with a sentiment of 'mission accomplished' that I leave the agency, which is perfectly structured today with a first-rate team and cutting-edge techonolgy", Stefani said in a company release.
"We’ve just launched a new version of Citizenside, probably the best connected site in France in terms of social networks, and will be soon revolutionizing the approach to news image sales with Editorside. The company has new exciting adventures on the near horizon, and I will stay a pro-active share-holder, and passionate about this sector."
Editorside is a new project from the site, which will allow Citizenside clients to view and buy unpublished photos submitted by Citizenside members to other titles. The site is currently in beta.
"After five years creating and developing Citizenside, Matthieu will leave his operational functions," Xavier Gouyou Beauchamps, president of Citizenside said.
"He will remain a refferal as one of our shareholders and we will count on him as a non executive partner. I want to thank him for his intense and efficient work, that has led Citizenside to become an international actor in the news business."
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