Pinterest also valuable for librarians & archivists.
"Pinterest is one of the newest and fastest growing social media networks out right now. Pinterest is a place to share pictures on "boards", organized places to store the photos that you "pin".
Since it came out in 2009, Pinterest has been an invite only site, making it exclusive and generally spam free. Although it is invite only, you probably know someone who is already a member and willing to invite you in. According to a report by Experian, the site received over 11 million visits the week of December 17th, which is 40x more hits than just six months earlier."