Not quite. Zenphoto "logon" status is cookie maintained. The cookie can expire. At that point the user will have to login again.
If he has Federated credentials he would logon via that provider. Different providers behave differently. For instance, with Google, if you are logged in to your Google account and select the "Google" provider at the Zenphoto logon you will get be get back a Google credential and be logged into Zenphoto with that credential.
There are two protocols that OpenID transaction use to get user information. Federated logon supports both. These protocols are used to request user information from the provider. We request the user's name and e-mail. Unfortunately the provider does not have to furnish this information. All that he has to do is supply an identity. We use the identity as the Zenphoto user id.
I do not know if Facebook is and OpenID provider. They operate as an OpenID consumer at least. If they are a provider then they will support all the above protocols.
Providers can have "discovery" protocols. (That is all that Google supports.) This is basically a URL that the OpenID consumer links to. That URL then figures out "who" the client is, verifies that the site is allowed to get his credentials (usually by asking on the first attempt and remembering the answer.) Then it returns an Identity to the consumer along with any of the requested data that it deems it will support.
Other providers require the full identity URL to validate a user. This is the generic OpenID protocol--what you use if you select the OpenID choice from Zenphoto. What the identity url is will be provider dependent. But it is ofthe something like https://<user id>.provider.com.
Verisign works this way. The Zenphoto Verisign handler first requests the user's Verisign user id then splices together a Verisign Identity URL and sends it off to Verisign for verification.
Your task then is to figure out what Facebook supports and build a handler for it. The Zenphoto Google and Verisign handlers should provide you with a model for whichever mecanism Facebook supports. If they have some third mechanism we will have to figure out how to deal with it.