Requisite OpenSocial post
Facebook invented, and Google have just generalized, an entirely new class of application. Take Dopplr. It's a social network that does trip tracking and coincidences. This is completely backwards - the social network stuff is overhead that we have to implement so that we can keep trips private and restrict the list of coincidences that you have to manage. Dopplr should really only be about the trips.
Facebook and Google have suddenly enabled a class of application that is the interesting half of this - I can think about the functionality, and sit in someone else's social network while I do it.
Paul Mison makes an interesting distinction between profile-centric and media-centric social networking services - Facebook being profile-centric and Dopplr, or Flickr, say, being media-centric. This evolution benefits both media-centric services (because they can avoid having to implement all that profile-editing overhead) and profile-centric ones, because they get events to drive return visits. Even heavyweight media items that take a bit of effort (a Flickr photo upload, a long-form blog post) tend to have lighter things attached (comments, usually), and these do very well at driving return traffic.