The problem may not be limited to online readers.
Have you ever heard of "tsundoku"? It's an informal Japanese word defined as "the act of leaving a book unread after buying it, typically piled up together with other such unread books."
This reminds me of a phenomenon which I observed among medical students and surgical residents over the course of many years.
Whenever a subject arose that they were not too familiar with, they would go off to the library and copy some articles about it and carry the articles around in their pockets for weeks. The papers would curl up at the edges and become as soiled as their white coats. But most of the time they were never read.
I would point out to them that photocopying an article, even though it can take a few minutes, was not a substitute for actually reading it.
I thought I might have been the only one to have noticed this, but recently a Twitter follower of mine, Terry Murray [@terromur], tweeted, "In the 1980s, the librarian at Hosp for Sick Children in Toronto urged 'neuroxing' (i.e., reading) instead of photocopying."
The Internet version of this phenomenon is facilitated by programs like Evernote, which make it easy to save links or PDFs for reading later. And you don't even have to go to the library.
I suppose some people eventually do read them. But I'll bet the majority don't.
Maybe the definition of tsundoku should be expanded to include the act of leaving a link unread after tweeting it, typically piled up together with other such unread links.