Firstly, The Onion does not report the news. It publishes fake news stories, on purpose, for entertainment.
One of the hallmarks of satire, the brand of humor The Onion practices, is that the satire has to be so absurd as to be obviously an exaggeration. The problem is that at a venue such as FoxNews or Fox Nation, its forum, satire is lost on its readers, because they are ready and willing to believe just about anything, no matter how ludicrous, just so long as it feeds into their conservative mob mentality.
On Friday, after The Onion published this story about how Obama was so depressed that he sent out a massively long email (150 pages!) to "the entire nation" (i.e., every American citizen with an email address), explaining once again all his positions on all the issues in great detail, Fox Nation ran the story without initially mentioning that the source was The Onion, publishers of fake news.
Fox Nation readers believed the story, and began writing comments, bemoaning the state of the nation, being run by such an obviously unfit person, proof being that Obama would send out such a ridiculously long email to all Americans. Only a few readers grasped that they were being duped by Fox Nation.
As the story got out, and other media outlets began to expose the truth about Fox Nation's embarrassing mistake (or effort to mislead), the site finally changed the headline of the story to reflect the source as being The Onion. Shortly after this however, Fox Nation pulled the story, and all comments, from its website.
Some of the comments, as well as an early Google cache showing the original headline (without any mention of The Onion), can be accessed here.