As the Google OpenSocial train pulls out of the station (see Techmeme)with some passengers on board, we are waiting for the reaction of those who have chosen not to go along for ride at this point. Major Facebook developers, such as iLike and Slide, have jumped aboard.
Creating a more universal platform for developing applications and widgets (gadgets in Googlespeak) is something that Google is well positioned to do. The cost to play is low for developers, and they have an addressable market of about 100 million non-Facebook social networkers.
Facebook and MySpace have tremendous momentum (unlike Google's Orkut), but it's a matter of time before they offer support on their platforms for OpenSocial apps, and not necessarily at the exclusion of their more proprietary APIs, which will be ahead of whatever is done by committee--although it's not clear how much influence non-Google entities have on the OpenSocial specs. What OpenSocial is to Google, Java is to Sun.
If great applications show up based on OpenSocial, Facebook and MySpace will have to reconsider their more closed approach. But lots of buzz about OpenSocial apps on Orkut or LinkedIn isn't likely going motivate Facebook or MySpace members to abandon ship. It turns out that a long tail of applications doesn't necessarily entice people to join a social network. Facebook, for example, is growing fast because it has hit a nerve, just as the iPhone, with few applications but a great browser and user interface, did.
In parallel with OpenSocial, and exposing user profile, social graph and feed data, is the targeted advertising opportunity. This the real treasure that Facebook/Microsoft and Google will fight over.