The Facebook API generates one key that you can use for web, iOS and Android. Most of the interaction should be done within the app itself for the authentication of the user. Once that's done, Facebook will return a unique user token that you could use to lookup that user in the jfbconnect_user_map table and find their Joomla ID. After you have that Joomla ID, you know what user you're working with and can fetch any other data you need.
So, in short, JFBConnect won't be doing the authentication within your app, you'll just do the database checks to find the user within JFBConnect and find their Joomla account. Conversely, if you need to create the user through the app, you need to create the user in Joomla and then add the proper row to our user_map table so we recognize them in the future.
If you're building an app and planning a whole service to do this, the above (hopefully) doesn't sound too scary
As for a webservices extension, unfortunately, I don't have any good recommendations for you. With so many extensions and unique cases, if you need a webservice, a lot of the time, it's roll your own.
I hope that helps,
Alex
mobile apps & jfbconnect support
