I don’t think you article really addresses the phenomenon referred to in the title, because the three reasons are not specifically associated with spreading ‘junk news’, they are the reasons any inform ation spreads. As for junk news:

There’s a prologue to Chris Brookmyre’s novel “Not The End Of The World” in which the author presents us with a character named Joey Murphy, captain of a fishing boat operating off the west coast of the USA by depicting Joey, out on the ocean, with nothing but salt water visible all round him and telling the readers that joey believes in UFOs, Alien Abductions, The Illuminati, Bigfoot and a list of urban myths, all of which he knows aren’t true, but believing in them makes life more interesting.

My friends and I, all professionals but what might be described as non conformist thinkers, would, before age slowed us down, argue for and against intelligent design. None of us are even religious let along creationists, but we’re not militant atheists either. Take away those elements and you have a subject thatcan be pulled, stretched and twisted in almost any direction, simply for the fun of it.
And there you have why junk news spreads so quickly, it’s more entertaining than realism.