Up to now, I refrained from posting anything on the FB “contagion” issue, but this article seems to have the right approach.

Facebook has made no secret of the fact that its news feed is a manipulated version of reality. It selects the posts and links to display prominently that it has found through testing are the most likely to interest users, and encourage them to return and post themselves. These tests are not sinister experiments; they are product development. All true, but there is one big difference: we are the product that Facebook has been testing. — FT.com.