Web searches seem to be full of results like “Top 5 Ways to <something>” or “Ten Ways that <someone> can Improve their <something>”.
It’s like they have all received some great wisdom that “top ten”-style lists are the way to attract readers. Is that it?
That’s it. People like lists and articles headed “top X ___” get more clicks.
WAG: It’s partly a fad, partly a useful handy way to structure an article, and partly a way to get people to click past the first page because it makes it obvious there’s more and makes people curious what the more is.
I don’t think it’s a fad; it has been around forever (long before the internet). That said, I could see an eventual backlash of sorts. List-based articles aren’t favored as much by sites like the NY Times.
Your statement that a list makes it obvious there’s more is right on. See Blogging: The Ultimate Guide to Content That Works [2023]
2006, eh? (I got the date from the comments, because, thanks to another of my pet hates, there was no date on the article itself). Well, maybe I’ve been blind to it before. But it seems to be everywhere now. Or maybe I’m just reading crappier web articles these days.
Area housewife discovers this one simple trick to get people to click through to her web page. 
I definitely see it a lot more now. I think some of it because there truly are more of these posts, but another big reason is the sharing of content and cross-marketing between sites. Sticking to a more traditional news sites (like CNN) in the past meant not seeing as many of these. However, most of these news sites now include links to more tabloid-like sites.
Right now I’m looking at a news story on CNN.com, and at the bottom are links to articles on Reader’s Digest, E! Online, Radar Online, Vogue, and Digestive Medical Solutions (Irritable Bowel Syndrome: Finally, Answers That Reveal and Treat Causes - wait, answers you say?! I must click!).
Aside from driving clicks, I think they’re also popular simply because they’re easier to churn out the more fully written articles. Just coming up with the title (and hence, what your going to make a list of) is basically half the work.
Since the internet has made the marginal cost of distributing articles close to zero, there’s a lot of websites that specialize in “quantity over quality”. For them, a format that makes it easy for a writer to churn out an article in a few hours is a naturally going to be over-used.
Is there something about giving a specific number of items that makes an article more attractive to potential readers?
Take an example from the link above: “12 Secrets of Better Lawn Care”. Why is that more effective than the un-numbered “The Secrets of Better Lawn Care”? Both are likely to be spread over several pages, both suggest that there is more to come (i.e. the factors given above).
It also provides some context. If I make a list of the five best Presidents, I’m not only commenting on the five men I chose but also, by implication, on all the others I didn’t choose for my list. If I commented on five randomly chosen Presidents without presenting them as a pattern, it tells less about how I feel about the Presidency overall.
A specific number sets expectations. A reader can think “sure, I have time to read 10 items.” People are less willing to invest their time if they don’t know the commitment.
The first *Book of Lists *by Wallace, Wallace, and Wallechinsky, came out in 1977. It was a huge bestseller. I think that was the first indication of how much the public loved lists of stuff.
OK, sounds plausible. So has this theory been tested?
It makes it easier to comment on the article also. Now you can “comment by number” and write your comment without spending as much time providing context (explaining to other people which specific point you’re commenting on.) Just say, “regarding number 5, I think…”
Yes, but you can do all that without having to say that there are five of them. Your article could simply be entitled “The greatest presidents”, and then 1. Warren G. Harding 2. Franklin Pierce 3. James Buchanan etc. (YMMV). No need to entitle it “The five best presidents”.
All of the above is true. Yes, it is a fad. Yes, the use of numbers and lists has been around forever. Yes, people do click more because of them. Yes, they do pander to the societal ADD the online world has developed. They’re not mutually exclusive.
Sites will continue to pummel us with these pages until they stop working. Then they’ll figure out what alternative works and drive that into the ground. It only has to work in the short-term.
I’m not finding specific studies about the importance of numbers, but here are a couple articles that refer to the theory:
http://moz.com/blog/5-data-insights-into-the-headlines-readers-click: Take a look at the comparison of “30 Ways to Make Drinking Tea More Delightful” (36% preference) to “Ways to Make Drinking Tea More Delightful” (15% preference).
http://www.jonathanfields.com/blog/irresistible-headlines/: he references the book Made to Stick by Chip and Dan Heath and comments that it says numbers are one of the critical elements that makes a message memorable. I haven’t read the book but presume there are more details on studies in it.
I’ve followed the development of cracked.com for years and I think has to do with light reading. Its easily digested, scannable and makes it more visually pleasing and less intimidating. Of course Cosmo and the like have been doing this for forever, but its still for the same general reason.
A list of ten things on ten web pages means ten times as many ads than a single page.