Re: 2. Two social sites are clear winners: Facebook and Twitter. Delicious initially had strong adoption, hence they were acquired, but it's been downhill since

Re: 2. Two social sites are clear winners: Facebook and Twitter. Delicious initially had strong adoption, hence they were acquired, but it's been downhill since then. Google Buzz can be considered a failure; Google forced their social graph upon users and they weren't happy. For the two successful sharing sites out there, hundreds are struggling (Digg included) or dead. In regards to the rest of your points, there's no telling how the product will exactly play out until we have access to play with it for ourselves.