This is a really good question. I don't know of any really large (e.g. consumer-oriented, Web 2.0) sites that use continuations. Many of the largest sites have specifically adopted simple REST APIs to provide themselves the highest scalability. It's my belief
that most of the sites using continuations are smaller, corporate sites that have a known, limited user base and can afford the luxury of a continuation-oriented web design. That said, I don't know that for sure. I'd be very interested in hearing any counter-examples, where people know of a consumer-oriented site that is using continuations in any big way, and in particular if it's scaling well.
I wrote about this exact topic in a couple of posts on my blog. You might find them interesting. I had at least one discussion with Marco Baringer, the author of UnCommon Web, about whether continuations scaled. Marco was for it, at least for places where it worked. I was skeptical and thought you'd run into problems sooner, rather than later. A lot of it comes down to what the word "scale" means to you. If that's 1000 concurrent users, then something like continuations probably works. If that's 100,000 concurrent users, then it probably doesn't.
Personally, I come from a hardware background and I tend to think in terms of state machines anyway, so the advantage of being continuation-based isn't as big of a deal in terms of how I would structure a program anyway. For instance, frameworks like Rails or Django fit my mental model quite well.
Here are the links:http://www.findinglisp.com/blog/2004/11 ... story.htmlhttp://www.findinglisp.com/blog/2004/12 ... -rest.html