The site manager is jointly-responsible of the data processing of users if he inserts a third-party plugin in his platform (for example, the "like" option of Facebook), because in this way users data are collected and trasmitted.
These are the conclusions of the General Lawyer Michael Bobek regarding the Fashion ID case (a German company that inserted the "Like" botton on Facebook) discussed in front of the European Court of Justice (case C- 40/17).
In case of acceptance of this issue, the site manager must transmit the minimum informations about the data processing to the users.
In the absence of an explicit consent of the user, data processing is declared legitimate if three condictions subsiste: a legitimate interest follow by site manager or third party that receive the data, the data processing shall be necessary for the achievement of legitimate interest; the data processing must be realize with regard of the essential rights and freedom.
In any case, this position must also be evaluated in consideration of the new regulatory obligations indicated by the GDPR for the processing of personal data of European persons.
Our staff is available for any clarification.