In what I like to call "the real world" -ie, the place where no one has heard of commontag, Freebase, or Zemanta, and maybe not even gizmodo - the #tag is the closest you're ever going to get. People use it on twitter and instagram, and advertisers have embraced it. Do any of these giant companies want their users going to other sites? Hell no. Facebook brought back the walled garden, and open systems are going to suffer.
Now that we've realized it's unlikely to happen, would you even want it if it did? If you add an ubuntu link on pinboard, would you want to instantly see all the old ubuntu stories on slashdot? Tag a flickr picture with "hotdog" and see all the tweets about hot dogs? Or take a picture with some app that adds its own tag (#vsco or some such) and see all the other pictures taken with that app? Some of these things actually work, but why? I could see doing something like subscribing to only slashdot/bizarro or gizmodo/tv in your RSS reader, but take a look at the RSS market and no one really gives a shit about that either.
I think wide-area tagging is quasi-useless. Even in closed silos (twitter, instagram), it's a messy sea of miscategorization and gamification. If it helps out the sites search engine, great. If it helps your own organization in whatever tool, great. It may even be good in workgroups - i'm interested to see how it pans out in OS X Mavericks.
