There are articles, studies and commentary on this issue all over the Internet. Here is an interesting graphic which tries to argue our attention span is slowly diminishing as media and the way we consume content gets easier and more efficient.
John Cecil of the Online Video Revolution channel on YouTube, explains video attention spans:
Our attention spans on TV is short too:
According to adexchanger.com, Morgan Stanley recently pointed out, "...28% percent of consumers time is spent online yet only 13% of ad spend is there. The percentages are out of whack by approximately $50B. Contrast that to TV where consumers spend 31% of their time but garners 39% of ad spend." It is clear getting consumers eyeballs on your site is both important in terms of hits, in terms of page clicks and in terms of time spent on a given site.
The ability for a website to grab attention and keep attention should be the number one priority for any content producer. You would think content itself would grab this attention, but it seems there is so much more involved.