Yahoo has concluded that Tumblr, the social blogging service, is worth a whopping $1.1 billion. Will the bet prove a good one?

The answer depends on whether the combined company can turn its many millions of users into a interlocking set of services that no competitor can hope to replicate.

In short, the question is whether Yahumblr, or Tumbloo, or whatever we want to call it, is more like Google or more like Craigslist.