What is best practice for Node.ACS to access logged on Facebook user from a mobile app.

Let's assume I have this basic architecture for a mobile app:

• titanium - mobile app
• node acs - express webservice as a custom backend for the mobile app
• acs cloud to store objects

I log in a facebook user in the mobile app and submit that auth to the ACS cloud:

Cloud.SocialIntegrations.externalAccountLogin({
        type : 'facebook',
        token : facebook.accessToken
    }

I then do a request to the Node.ACS webservice for something but of course the Node.ACS is not aware of any existing session with the ACS cloud. What is the best practice to handle sessions from a mobile app and facebook in Node.ACS?

Thanks!!