It's been less than a year since Yahoo acquired "social bookmarking" site Del.icio.us. At the time of the acquisition, Del.icio.us had 300,000 registered users. Last night, founder Joshua Schachter announced that Del.icio.us had officially hit the one million user mark - a cause for some minor stir in punditville.

"Think only the elite opinion leaders of the tech and online worlds are using social bookmarking services such as Yahoo’s Del.icio.us?" asked Search Engine Journal. "Think again." Indeed, 200 percent growth in less than a year suggests social bookmarking is more than a passing fad, and it suggests that Yahoo's ambitions to pioneer social search have not faltered.

That said, with the proliferation of "social" sites of various flavors since the early days of Del.icio.us and Flickr, the boundaries between Web 2.0 innovations seems increasingly fuzzy. One sometimes wonders, for example, where a site like Del.icio.us ends and a social news service like Digg begins - and whether a massive consolidation is likely to happen someday soon. The Browser put this question to Mr. Schacter yesterday when he stopped by for a chat, and he argued the distinction was fairly clear: Del.icio.us is a first key step in social search.

"Your attention over time is memory," he said, and social bookmarking is a way to document that attention such "that it is accessible to other people." While social news sites like Digg help people filter today's news, they don't act as a collective memory. "Social news gets thousands of new items a day. We get hundreds of thousand of new items a day."

On a more practical note, when asked whether the ad-free Del.icio.us was making money, or would soon, Schachter seemed unconcerned. His job is to focus on the site's utility, he says, and the money would take care of itself. Or at least that was some other Yahoo's job.