Suppose I'm a tenured professor and I publish a blog with educational posts about computer science.  I write blog entries during my working hours.  The blog isn't a requirement of my job but it may help raise my academic profile.  Am I free to use your CC-BY-SA 3.0 photo to illustrate my blog post?<p>What if, instead of a professor, I am a freelance programmer.  There are no ads or other exchange of money directly related to the blog, but sometimes potential clients find me through my blog.  But the blog is also a way for me to converse with a group of regular readers whom I already know.  Is this blog <i>primarily</i> directed toward commercial advantage?<p>In either of the above scenarious, suppose the blog has ads but that I don't turn a profit on it (i.e. the ads defray but do not cover the hosting cost).  How about now?<p>Or what if the revenue from the ads is more than the hosting cost, but much less than I could make through my professional activities.  If the amount of money is tiny relative to the time I spend on the blog, could I argue that the blog is not <i>primarily</i> directed toward commercial advantage?<p>You may find each of these cases perfectly clear and obvious, but wherever <i>you</i> draw an "obvious" line, the above study probably shows that many Creative Commons members will think it should be somewhere else.  Even the organization that drafted the license admits that it doesn't provide very explicit guidance for common cases like web sites that include (but don't rely on) advertising, or sites that do not involve exchange of money but may provide less-tangible commercial advantages for their owners.