A guide written by the pioneering usability experts Jacob Nielsen and John Morkes famously said this in 1997, 12 years ago:

Make your writing concise, SCANNABLE, and Objective.

A summary of the article:

- Users want to search for things online.
- They hate waiting.
- They read from left to write in a 'Z' fashion.
- Most readers scan over headlines without bothering to read the article.

So, you got less than a second to get the reader's attention.

"... any story can be told in 800 words" - Roy Peter Clark, senior journalist, Poynter.org
This is as good a guideline for online writing that you can get anywhere.

More important observation on online user behavior Jacob Nielsen's highly useful Useit.com site is as follows, but I suggest you jump to each link and read for yourself in detail:

- Users spend 4.4 seconds for every extra 100 words on a page.
- Eye tracking visualizations show that users often read Web pages in an F-shaped pattern: two horizontal stripes followed by a vertical stripe.
- You can use passive voice in headings.
- Contrary to popular opinion, long articles may work , if they have something of value, are divided into short paragraphs with descriptive paragraph headings and augmented by lists, charts, and photos.i
- Write in an inverted pyramid style: start with the conclusion and put the least important data lowerd down.
Start subheads, paragraphs, and bullet points with information-carrying words that users will notice when scanning through your article in a "F" shaped pattern.
- The first two paragraphs must state the most important information.