Let me start this post with two disclaimers. First of all, I think OpenSocial is a great idea, and I'm excited to start building applications for it. Second, I Am Not A Lawyer.
I wasn't satisfied. With a grandiose name like OpenSocial, I thought there has to be something inherently better in here. So I started digging into the Terms of Service. In a truly Open world, an application would be able to move a user's data seamlessly from one site to another (with their permission of course), making one big happy world. Again, IANAL, but from what I can tell, that is not the case. Here is the relevant section:
If your gadget enables you to gain access to photographs, images, videos, music, audio files or other sounds available on a Google website, you may use that content solely to display or provide it to the user from whom the content is obtained. Other websites may have policies regarding your use of or access to content of various formats, and it is your responsibility to make yourself aware of and comply with those policies.
Contrast this with the Facebook TOS:
4) Except as provided in Section 2.A.6 below, you may not continue to use, and must immediately remove from any Facebook Platform Application and any Data Repository in your possession or under your control, any Facebook Properties not explicitly identified as being storable indefinitely in the Facebook Platform Documentation within 24 hours after the time at which you obtained the data, or such other time as Facebook may specify to you from time to time;
Both of these are pretty dense, but it sounds like the story is about the same. You can store some basic information, but the various sites have control over what you can export. My dreams of a unified photo album where you can add pictures from any site and view them from all, are pretty much flushed down the toilet. At least Facebook is kind enough to define the data we can store, Google remains vague about it.
And lastly, Google retains the right to kick you out without notice. This was one of the big fears in developing a Facebook application, and it looks like it won't go away. In the end, these are all businesses, and while they like the PR that goes with "Open", they're not going to do anything that doesn't make business sense, or give away much of their power. OpenSocial is a business decision, by pooling their sites, they all get more application development, and a side effect is that it makes a developer's life a little easier.
In summary, I don't see a very compelling case the OpenSocial is going to put the Facebook API out of business. I'd love to hear other people's thoughts on this, especially if my reading of the TOS is wrong, which it very well might be.
UPDATE 11/3: Just read gwachob's post about OpenSocial, and I think his analysis is very similar to mine. His writing is much better, though, so go read it :)