Facebook ShareLink official example code not working

The error message under the callback system is:
“The name `String’ does not exist in the current context”
What am I missing? Code example below:

FB.ShareLink(
    new Uri("https://developers.facebook.com/"),
    callback: ShareCallback );

private void ShareCallback (IShareResult result) {
    if (result.Cancelled || !String.IsNullOrEmpty(result.Error)) {
        Debug.Log("ShareLink Error: "+result.Error);
    } else if (!String.IsNullOrEmpty(result.PostId)) {
    // Print post identifier of the shared content
    Debug.Log(result.PostId);
} else {
    // Share succeeded without postID
    Debug.Log("ShareLink success!");
} 
}

Add using System at the top of your script.