One of these things is not like the others
Please go to the Question #6 post if you haven't had a chance work on this one yet, since the text below contains the solution.
On the days that I post a new puzzle, I also send out a message on Twitter announcing the new question. In this particular case, the readers who were directed here through Twitter had a large advantage, since that is the key to the answer. Three of the items are associated with Twitter. Mark Zuckerberg isn't -- he's the CEO of Facebook.
Ashton Kutcher was in the news recently for being the first Twitter member with 1 Million followers (he's well past that now, and still has the most followers). Hashtags are words or phrases added to messages which tag them as being on certain subjects. And the fail whale is the image that you get when Twitter is (far too often) unavailable.
Since Mark Zuckerberg isn't associated with Twitter the way that the other three are, he doesn't belong. He is not like the others.