Urged by the explosive growth of Facebook, Google and a number of partners have unveiled the OpenSocial APIs that were rumored for some time. Pretty interesting stuff that will allow the development of portable widgets (or gadgets as Google calls them).
My feeling is that Google doesn't aim to be a major player as a hosting platform. Their Orkut was one of the first social networks (I was invited when it opened in 2004) but is not really successful, except among brazilian girls :-)
So their interest is most probably in the creation of a large ecosystem of social networks, which will use Google ads as their primary revenue source. Don't compete directly with Facebook, but feed those who will.
The interesting thing is that it will allow an intermingling of the currently many isolated social networks we're currently subscribed to. Containers and applications can now become more or less orthogonal concerns. The container will provide a "color" to the network (its purpose) which will define the kind of people to whom it will be of interest. And then, those people will be able to install whatever applications they like in their network.
Actually not far from how we choose our place in the real life: people choose a district (the network or container) that provide the infrastructure and a general atmosphere, and then go shopping for furnitures (the applications). Of course, the district somehow defines the kind of furnitures you buy, but nothing prevents people from Beverly Hills to go to Ikea!
I think however that saying "write once, run in all containers" will not be the exact truth. Applications will have to be "tailored", at least in terms of presentation style to the containers to ensure a smooth integration in their environment. The sample showing the Flixster app in the Ask a Ninja Fans network is ugly: a white rectangle in an otherwise black site. So application writers will have to target the kind of network they address. But here also, nothing new: furnitures come in different colors so that everybody can find the one they like!
Interesting times ahead!