这是Facebook出于安全考虑而设计的。下面是Facebook团队成员埃里克·奥斯古德的解释:

This has been marked as 'by design' because it prevents a potential security vulnerability. Some browsers will append the hash fragment from a URL to the end of a new URL to which they have been redirected (if that new URL does not itself have a hash fragment). For example if example1.com returns a redirect to example2.com, then a browser going to example1.com#abc will go to example2.com#abc, and the hash fragment content from example1.com would be accessible to a script on example2.com. Since it is possible to have one auth flow redirect to another, it would be possible to have sensitive auth data from one app accessible to another. This is mitigated by appending a new hash fragment to the redirect URL to prevent this browser behavior. If the aesthetics, or client-side behavior, of the resulting URL are of concern, it would be possible to use window.location.hash (or even a server-side redirect of your own) to remove the offending characters.

来源:https://developers.facebook.com/bugs/318390728250352/

不知道他们为什么这样做，但是，你可以通过重置页面顶部的哈希来解决这个问题:

if (window.location.hash == "#_=_")
  window.location.hash = "";

这是Facebook出于安全考虑而设计的。下面是Facebook团队成员埃里克·奥斯古德的解释:

This has been marked as 'by design' because it prevents a potential security vulnerability. Some browsers will append the hash fragment from a URL to the end of a new URL to which they have been redirected (if that new URL does not itself have a hash fragment). For example if example1.com returns a redirect to example2.com, then a browser going to example1.com#abc will go to example2.com#abc, and the hash fragment content from example1.com would be accessible to a script on example2.com. Since it is possible to have one auth flow redirect to another, it would be possible to have sensitive auth data from one app accessible to another. This is mitigated by appending a new hash fragment to the redirect URL to prevent this browser behavior. If the aesthetics, or client-side behavior, of the resulting URL are of concern, it would be possible to use window.location.hash (or even a server-side redirect of your own) to remove the offending characters.

来源:https://developers.facebook.com/bugs/318390728250352/

我使用这个，删除'#'符号。

<script type="text/javascript">
    if (window.location.hash && window.location.hash == '#_=_') {
        window.location.href = window.location.href.split('#_=_')[0];
    }
</script>

我看不出这个问题与facebook AJAX有什么关系。事实上，禁用JavaScript和完全基于重定向的登录也会出现这个问题。

一个与facebook交换的例子:

1. GET <https://www.facebook.com/dialog/oauth?client_id=MY_APP_ID&scope=email&redirect_uri=MY_REDIRECT_URL> RESPONSE 302 Found Location: <https://www.facebook.com/connect/uiserver.php?[...]>  
2. GET <https://www.facebook.com/connect/uiserver.php?[...]> RESPONSE 302 Found MY_REDIRECT_URL?code=FB_CODE#_  
3. GET MY_REDIRECT_URL?code=FB_CODE#_  

我也只在火狐浏览器上遇到过这种情况。

适用于PHP SDK用户

我只是通过在转发之前删除额外的部分来解决这个问题。

 $loginURL = $helper->getLoginUrl($redirectURL, $fbPermissions);
 $loginURL = str_replace("#_=_", "", $loginURL);
 header("Location: " . $loginURL);