Kevin Cawley points out a few potential flies in the ointment of the soon-to-be-announced OpenSocial API. In a particularly apt analogy he reminds us of the early promise of Java: write once and run anywhere. The reality was that while Java turned out to be a perfectly fine programming language, there was no way to abstract out all the differences among platforms while still taking maximum advantage of the unique capabilities of each. The same thing is likely to happen as developers attempt to adapt their applications to the multiple social networks that will be behind the OpenSocial API.
Cawley's prediction is that Orkut will enjoy a momentary increase in usage as developers use it as a sandbox but then those same developers will get bogged down in trying to expand their app to the other platforms. I am less concerned about that - any developers who sanely wanted to diversify across multiple platforms would have to write their own abstraction layer if they didn't use OpenSocial - it will be interesting to see how closely these competing SNS companies will adhere to the emerging standard.