The Mendeley.com library of references is crowd-sourced. All references added to personal Mendeley libraries are discoverable by all Mendeley users. This is what drives the discovery aspect of Mendeley. Note: all citations are anonymized, so your citations will not be tied to your name or account.
If you don't want a reference in your personal Mendeley library to be added to the crowd-sourced library (unpublished papers, personal manuscripts, class notes, e.g.), highlight the reference in your desktop library and at the bottom of the right column, check the box next to "Unpublished work."More information here.