Obviously ads. Generally I prefer single page (+ Readability), but was surprised that sometimes I prefer multiple pages. I think it's when they act a bit like chapters, giving you a sense of completion. Maybe it's when the content is dense and/or boring. It also makes it easier to "hold your place", if you need to do something else (like research something in the article). I'm one of those people who highlights text to keep my place on the page (so I have to adblock some sites' JS that hijacks highlighting (e.g. NYTimes)).