I've now had the time to reflect on what Google's Open Social means, and look at some of the posts on Digg about it, and the more I think about it, the more convinced I am that this is going to completely change the social networking landscape. Before, I was a bit uncertain, but now that MySpace is participating, all the major players except for Facebook have signed up. Despite the fact that Facebook is on the up, MySpace alone still outnumbers them. I believe that Bebo is also quite large, and Orkut is big (it's just not popular in the US and Europe, in India and Brazil it's huge).
This puts Facebook in a difficult position - a new industry standard for designing applications has sprung up almost overnight, and they're the only major player not using it. This means that in terms of applications, they're likely to become increasingly sidelined unless they adopt the new standard.
As a supporter of Free and Open Source Software, I'm all for Open Social. Proprietary walled gardens just lead to problems with interoperability. For instance, instant messaging is a pain because every provider has their own protocols and they don't generally interoperate - that really gets my goat! If your friends are all on MSN, you have to use MSN to talk to them, and if one is on AIM, you can't talk to them. Imagine if e-mail worked that way - it'd be a nightmare.
Time will tell as to whether Facebook chooses to participate or not - word is that they have been approached by Google to discuss it.