Thanks to Ian Davis I came across this list from Marc Hedlund of criteria for calling a platform open. While it was written in the context of comparing the iPhone with the Treo, it is equally applicable to the current discussion about the openness of Facebook.
Extracted from four paragraphs of Marc's original post:
An open platform
- allows developers to implement functionality the platform provider hasn't gotten around to yet
- allows developers to reimplement and replace functionality the platform provider has gotten around to, but has failed to do well.
- allows developers to meet needs that scare the platform provider, and allows consumers to have those needs met where otherwise the platform provider would block a capability
- allows its users to get far more done, and latches them to that platform far more tightly as a result.
Point #3 is often the hardest lesson for platform providers to learn, but if they do so they can reap the benefits of point #4.