I actually worked on the spurl search engine once upon a time... but that was a while ago! I haven't heard that name in a long time. It was a great service, but at the time there was quite a bit of saturation in the web-based bookmarking market. Furl looked like the clear leader, at the time, but in the end I guess it was del.icio.us with its simplicity coupled with social underpinnings. Pinboard has emphasized the former, focusing on raw, simple utility. I think it is here to stay -- they seem to know what they're doing, on a technical level but as always, in the long run, we'll see.