Facebook to my knowledge has the largest number of social application developers, being the industry leader in the "social web platform" space. I was never meaning to imply Myspace doesn't have the most traffic, and more to your point, they are only participating, not governing. Myspace is still planning its own platform, as is Friendster and others.
My critism still remains that in not inviting Facebook (or Myspace, or anyone) to help *govern* the API, they clearly had not intended it to be "open". Standards organizations exist for a reason, so many people can come together and decide on whats best for everyone.