When you get referred by a contact to another or recommended for a project, it is advisable that you mention who recommended you UNLESS you were specifically asked not to.
By acknowledging your source, you'll be leveraging on an existing trust and the transactions would be made easier. That gesture also helps to strengthen ties between your contacts.
Submitting your CV/Cover letter follows the same principle, your recruiter will be interested in knowing how you heard about the vacancy. It will be satisfactory to include this detail in the introductory part of the body of your mail.
Sample:
______________________
Good day Sir,
I am writing to express my interest in the Graduate Engineering Programme as advertised on your company's official website. . .
_______________________
Dear Ma,
Kindly find attached a copy of relevant documents for the contract role as requested in the shared publication on Social Media (Facebook) through @Frank. . .
________________________
Best RegardsβοΈ
Thanks π for sharing