> In the absence of the authorisation referred to in the second subparagraph of paragraph 1, Member States shall provide that an online content sharing service provider shall not be liable for acts of communication to the public or making available to the public within the meaning of this Article when:

> (a) it demonstrates that it has made best efforts to prevent the availability of specific works or other subject matter by implementing effective and proportionate measures, in accordance with paragraph 5, to prevent the availability on its services of the specific works or other subject matter identified by rightholders and for which the rightholders have provided the service with relevant and necessary information for the application of these measures;

So basically if the rightsholders can convince the courts that they could have done more to proactively prevent copyright infringement, they're liable.