The crux is “If you are linking to your own site, make it dofollow, if you are providing opportunity for other users to link to their own sites or to other sites (those links that are not editorially approved) should be marked as nofollow”
Now, have a look at the video of Matt Cutts
Also See:-
Rel=nofollow Links for Widgets and Infographics
Syndication is a Valid Way to Get Links
Google Pushes for User Sharing of Sites
Where to link to the original source in the article?
Links from article marketing sites
Importance of Backlinks in Seo
How to Create Backlinks That Google Loves
How to Rank in Google
Google Search Quality Updates
Social Media Widgets for Websites
Nofollow Vs Dofollow