Regarding the no follow tag. When should it really be used?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  Sandra Sinclair: Hi Team - TGIF! =)
  
  Regarding the no follow tag... When should it really be used?
  
  For instance, on my website, should the no follow tag be used for the social media piece? no follow to FB/linkedin/g+/twitter/IG etc...
  
  Someone please shed som light - im going crazy with this tag! hehe
  
  Best,
  Sandra
• 
  Tony McCreath: I'd say there is no reason to nofollow them. They are not paid links or links generated out of your control.
• 
  Robert Heine: There ARE Reasons for this. Mostly on pages where you list several other pages. Everybody wants to be on this list, if it is a normal link (follow), because: Backlink-optimization.
  
  But if you put a no-follow to it, only those that really need to be on that list will put their link. No useless link-list will be created.
  
  Their were some social-bookmark-services that used this technique. People created their bookmark-lists, set them to "public visible" and hoped they will get a lot of backlinks from the search-engines. But as the no-follow-attribute for a-tag was used, they didn't ;)