iOS Facebook login, handle different access tokens and determine users identity

iOS Facebook login, handle different access tokens and determine users
identity

I have a couple of questions regarding facebook integration and
authentication in my social networking iOS application.
First of all, Users of my application are required to create an account
and login before any content is available to them.
I have a separate backend system, so regular account registration inside
my application is supported. When a user is logged in to an account he or
she is supplied an access token which is used for future backend-requests,
such as image-uploads.
Secondly, A user can choose to log in with Facebook, and thereby acquire
an access token from the successfully opened FBSession.
This leads to my first question: How do I use the access token acquired
from Facebook to authenticate my users with my backend system? Should I
send the acquired token to my backend and have the system use that token
for this particular user until further notice, or is there another way to
go about this?
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Now lets say that I solved the problem in the first question...
My second question: Imagine that a user created an account and logged in
the normal way (i.e. without facebook) The next time that user wants to
use the application, he or she chooses to log in with facebook.
Now, In my backend-system, how would I know that these two users are the
same, connect them and thereby be able to load the same content for both
cases? Is it even possible to have hybrid accounts that supports double
logins like that? A regular registration only requires email, username and
password, so I cannot think of a way to determine that a
facebook-loggedin-user is the same user as a normal-loggedin-user.
I Hope the above makes sense.