Force Facebook Logout

    // force close
    [[FBSession activeSession] closeAndClearTokenInformation];
    [[FBSession activeSession] close];
    [FBSession setActiveSession:nil];

    // remove FB cookie
    NSHTTPCookieStorage *storage = [NSHTTPCookieStorage sharedHTTPCookieStorage];
    for (NSHTTPCookie *cookie in [storage cookies]) {
        NSString *domainName = [cookie domain];
        NSRange domainRange = [domainName rangeOfString:@"facebook"];
        if(domainRange.length > 0) {
            [storage deleteCookie:cookie];
        }
    }