Best analysis i have found so far about the success Pinterest is dealing with. Note that they are still as of today, in private beta.

The goal is a simple one, you ideally want your users to be able to browse through hundreds or even thousands of images with relative ease.

One important step in this battle was “masonry” style layout, a fairly recent trend in web development named after the jQuery Masonry plugin. Pinterest uses its own scripts for this, but the concept is the same.

Websites love pagination because it increases their page views (more views = more money), but as a user it sucks [...] To solve this problem, the Pinterest team looked to infinite scrolling [...]

It’s interesting to note that the Facebook News Feed, the epitome of website addiction, uses infinite scrolling.

Pinterest on the other hand, has actually found an audience largely with females.

[...] in order to get people to actually use your digital content collection service, you have to make the act of collecting and sharing content a nearly effortless task. [...] This creates a system where collection is super easy and encourages lots and lots of pinning.