Facebook might be bigger, but I wouldn't call OpenSocial a failure since around a hundred million people are using it.

I've been writing an app that is supposed to work both on MySpace and Mixi.jp (~20M users), both use OpenSocial. While theoretically you are supposed to be able to write once and run everywhere, in reality you have to tweak your app to handle differences in feed publishing details, checking call signatures etc.

Still it was a lot nicer than having to write the app totally from scratch for each.