I submitted this point as another thread (http://news.ycombinator.com/item?id=3027328) but one interesting detail is that in order to share music, users must choose the same backend provider. I.e., if I use MOG and you use Spotify, we don't get the social features. In practice this is likely to mean one vendor will prevail at the expense of the others -- so being an unpopular partner may not be much better than being a non-partner in a year's time...