Updated, with new information about Creative Commons, on 27 Sept. 2015.

During a class on how news organizations use social media, one of my students asked if it was OK to republish any photo that appeared on Facebook, Twitter, or Weibo, the Chinese Twitter. 

Ellyn Angelotti,
Poynter Institute
There is no simple answer to this question, but there are some guidelines, as I learned during an online webinar, "Navigating Copyright and Fair Use Issues in an Open-source World," offered on NewsU by Poynter Institute faculty member Ellyn Angelotti.  

The legal concept of "fair use" in the U.S. and many other countries means that copyrighted material can be reused or republished as long as it meets certain criteria, Angelotti said. Copyright aims to encourage creativity and innovation by protecting a creator's work. But it also aims to encourage people to transform the original so that it advances public knowledge or creativity. 

 Some reuse is permitted

So, to answer the student's question, it is generally OK to republish a tweet in Twitter or a Facebook update within those social network systems. However, each social media platform has its own terms of use, which you should consult before republishing.
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