(Of course, few stopped to think about how Facebook decided what to show people in the first place, but that's beside the point.)
No, it's the entire point. Your stream, to the extent that it's "your" stream, is already manipulated in ways you're not told, based on what Facebook thinks you will find interesting, funny, or engaging enough to come back and see more ads. Experiments have to happen as part of Facebook's desire to expand, if only to see which manipulations mean more ads displayed. The only difference is that now the people who are interested in something closer to science than ad sales at Facebook won't tell us about their results. Are you happy?