Is it possible to use Facebook SSO with portal users?

You can use it with a portal (see step 11 in the documentation). The user’s Facebook ID is associated with their Salesforce user record. You receive their name and email in the registration handler when they SSO in.

The registration handler can use the incoming Auth.UserData to query for a match on an existing user and either create a new user or return an existing user as appropriate.

Here is a sample registration handler that illustrates the above:

public class PortalHandler implements Auth.RegistrationHandler{
    // createUser is called when there is no existing user linked 
    // to the incoming third party account
    public User createUser(Id portalId, Auth.UserData data){
        User u;

        // Use incoming email for username, since we're working with a 
        // portal user
        List<User> l = [SELECT Id FROM User WHERE UserName = :data.email];
        if (l.size() > 0) {
            u = l[0];
            System.debug('Found existing user record for '+data.username);

            // Update existing record
            u.Email = data.email;
            u.LastName = data.lastName;
            u.FirstName = data.firstName;
            // Useful to save the Facebook ID in a custom field
            u.Facebook_ID__c = data.identifier;

            System.debug('Updating user record for '+data.username);

            update(u);
        } else {
            // Portal users need an associated contact, which, in turn,
            // needs to be associated with an account.
            // For simplicity, just put all contacts on the sForce account
            Account a = [SELECT Id FROM Account WHERE Name='sForce'];

            Contact c = new Contact();
            c.AccountId = a.Id;
            c.Email = data.email;
            c.FirstName = data.firstName;
            c.LastName = data.lastName;
            insert(c);

            u = new User();
            Profile p = [SELECT Id FROM profile WHERE name='High Volume Customer Portal'];

            u.UserName = data.email;
            u.Email = data.email;
            u.LastName = data.lastName;
            u.FirstName = data.firstName;
            u.Facebook_ID__c = data.identifier;
            u.Alias = (data.username != null) ? data.username : data.identifier;
            if (u.Alias.length() > 8) {
                u.Alias = u.Alias.substring(0, 8);
            }
            u.Languagelocalekey = UserInfo.getLocale();
            u.Localesidkey = UserInfo.getLocale();
            u.EmailEncodingKey = 'UTF-8';
            u.TimeZoneSidKey = 'America/Los_Angeles';
            u.ProfileId = p.Id;
            u.ContactId = c.Id;

            System.debug('Returning new user record for '+data.username);
        }

        return u;
    }

    // updateUser is called when there is a match with an existing user
    public void updateUser(Id userId, Id portalId, Auth.UserData data){
        User u = new User(Id=userId);

        u.Email = data.email;
        u.LastName = data.lastName;
        u.FirstName = data.firstName;
        u.Facebook_ID__c = data.identifier;

        System.debug('Updating user record for '+data.username);

        update(u);
    }
}