Only I can sign into the app I have moulded in Objective C xcode. What are the possible causes of this?

I am using the latest version of FBSDKv4.0.1 in my app but only I can sign in to the app. When somebody else is trying to sign in error is obtained, why?

if ([FBSDKAccessToken currentAccessToken])
    {
        // User is logged in, do work such as go to next view controller.
        self.loginButton.readPermissions = @[@"public_profile", @"email",     @"user_friends"];
    ProgressHUD *HUD=[[ProgressHUD alloc]init];
    [HUD showTextOnlyAndHideForChat:@"Fetching details" parentView:self.view];
}

FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
[login logInWithReadPermissions:@[@"email"] handler:^(FBSDKLoginManagerLoginResult *result, NSError *error) {
    if (error)
    {
        // Process error
        NSLog(@"fbsdk error log");
        ProgressHUD *HUD=[[ProgressHUD alloc]init];
        [HUD showTextOnlyAndHide:@"Invalid credentials" parentView:self.view];
    }
    else if (result.isCancelled)
    {
        // Handle cancellations
        NSLog(@"fbsdk Handle cancellations");
        ProgressHUD *HUD=[[ProgressHUD alloc]init];
        [HUD showTextOnlyAndHide:@"Activity cancelled by user" parentView:self.view];
    }
    else
    {
        // If you ask for multiple permissions at once, you
        // should check if specific permissions missing
        if ([result.grantedPermissions containsObject:@"email"])
        {

            [[NSUserDefaults standardUserDefaults] setObject:[[FBSDKAccessToken currentAccessToken] tokenString] forKey:@"fbaccesstoken"];

           NSLog(@"fbaccesstoken %@",[[FBSDKAccessToken currentAccessToken] tokenString]);

//                NSDictionary *dictionaryJSON = [NSJSONSerialization JSONObjectWithData:result options:NSJSONReadingMutableLeaves error:&error];
//NSLog(@"DICTIONARY==%@\nresult===%@",dictionaryJSON,result);
            NSLog(@"fbsdk if log");
            // Do work

            ProgressHUD *HUD=[[ProgressHUD alloc]init];
            [HUD hideHudProgress];
            //------------test
            if ([FBSDKAccessToken currentAccessToken]) {
//                    ProgressHUD *HUD=[[ProgressHUD alloc]init];
//                    [HUD showHudProgressWithText:@"Fetching user information.." withView:self.view];

                [[[FBSDKGraphRequest alloc] initWithGraphPath:@"me" parameters:nil]
                 startWithCompletionHandler:^(FBSDKGraphRequestConnection *connection, id result, NSError *error) {
                     if (!error) {

#success statements#
}

This is the error log I am getting.

Error: Error Domain=com.alamofire.error.serialization.response Code=-1011 "Request failed: unauthorized (401)" UserInfo=0x7ac62bd0 {com.alamofire.serialization.response.error.response=<NSHTTPURLResponse: 0x79678040> { URL: http://ift.tt/1EbKJQy? token=(access-token-value)                                                                               { status code: 401, headers {
    Allow = GET;
    Connection = "Keep-Alive";
    "Content-Language" = en;
    "Content-Type" = "application/json";
    Date = "Sat, 25 Apr 2015 08:09:45 GMT";
    "Keep-Alive" = "timeout=5, max=100";
    Server = Apache;
    "Transfer-Encoding" = Identity;
    Vary = "Accept,Accept-Language,Cookie";
    "Www-Authenticate" = Token;
} }, NSErrorFailingURLKey=http://ift.tt/1EbKJQy?

NSLocalizedDescription=Request failed: unauthorized (401), com.alamofire.serialization.response.error.data=<7b226465 7461696c 223a2022 41757468 656e7469 63617469 6f6e2063 72656465 6e746961 6c732077 65726520 6e6f7420 70726f76 69646564 2e227d>}

Can anybody please tell me if i made a mistake in this code snippet? Or is it the bug in the latest FBSDK?