Set another page.tpl.php if a parameter is set i want to use another page.tpl.php file, if die fb parameter is set.
The File is located in my theme folder and has the name "page-fb.tpl.php"

My Function in template.php

function THEMENAME_preprocess_page(&amp;$vars) {
/** Facebook Output */
    if ( isset($_GET['fb']) &amp;&amp; $_GET['fb'] == 1 ) {
        $vars['template_file'][] = 'page-fb';
    }
}


The function is called, but the new page-fb.tpl.php is not loaded.
I'm doing all of this on Drupal 7.

Any advices?
Rename page-fb.tpl.php to page--fb.tpl.php and then modify your code to use $vars['theme_hook_suggestions'][] = 'page__fb'; instead of what you got currently. For further information you can check out the Drupal 7 Template Suggestions page.