From the Economist:
FRENCH internet executives, bloggers and web users cringed in early January when an independent report for the ministry of culture proposed a tax on online advertising revenues, aimed at American firms such as Google, Microsoft, AOL, Yahoo! and Facebook, to pay for new subsidies for the music, film and publishing industries. ...
To cover the cost the authors of the report recommended a tax aimed at the handful of American firms which dominate France’s internet-advertising market, notably Google, even though these bear no responsibility for the music industry’s woes. ...
...
But many French internet firms fear a new tax. It may be impossible to set a minimum threshold for taxable revenue that penalises foreign firms but exempts domestic ones, especially as the latter grow. If the limit is set low enough to ensnare Facebook, for instance, which earns comparatively little in France, some French firms could also be hit. ...
I'm not sure what GATS commitments the EU made for the relevant services, but if there were National Treatment commitments, this would make a great de facto discrimination case. If the article is describing the situation accurately, the law would involve a tax that is set at a level which catches "the handful of American firms which dominate France’s internet-advertising market." So, the measure itself does not identify American firms, but it sounds like there would be some good evidence, from both the effect and intent of the measure, indicating that this was what was going on.