Is it possible that if I have a user sign in with google, yahoo or facebook, that they I can post to their activity feed with the api (no email/password)?
At the moment members who have signed up for your Ning Site that haven't manually set a password cannot be authorized against the Ning API.
We are in the planning stages of implementing 3-legged OAuth. We are also planning to provide helper libraries to make signing in easier for native mobile applications (iOS and Android).
You may also be interested in reading a post a wrote a while back: Social Sign-In and the Ning API.