Why Facebook is not used by recruiters for sourcing resumes or for Networking?

Facebook is a social medium that aims primarily to promote friendship and personal connections, and not a venue for office work recruitment.

FB has a different purpose, i.e. non-professional. LinkedIn's much more appropriate for this

Facebook is only for personal communication, It is not recognized recruitment site. However, if anyone wants to use for recruitment, they can do.

Many recruiters use Facebook for making CV pool. Although its ratio is low as compare to LinkedIn because of the nature they both portals possess. 

Perfect answer provided by Miss. Nila