FACEBOOK  was created in February 2004 by a Harvard student as a social networking web site for college students to stay in touch.  It gained huge success immediately. Several months after FACEBOOK was invented, an important feature,GROUPS, was added for the convenience of sharing similar topics of interest among members.  Since then FACEBOOK groups have been heavily utilized.
Upon registration, any person can create a GROUP and invite members that are either just his/her friends, people in a network. 

FACEBOOK could be useful for libraries by notifying new library services and events and notifying members of new books.