Extracting a number from provided URLs inside a text file
You can use a regular expression to find the ID. The pattern [0-9]+$ will match one or more occurrences of 0-9 at the end of the string. You can use it like this:

private static readonly Regex UrlId = new Regex("[0-9]+$");

private static string ExtractID(string url)
{
    var match = UrlId.Match(url);
    return match.Success
        ? match.Captures[0].Value
        : string.Empty;
}




Instead of using a StreamReader, consider using File.ReadLines

foreach (var line in File.ReadLines(path))
{
    if (ValidateUrl(line))
    {
        TweetIDsList.Add(ExtractID(line));
    }
    else
    {
        Logger.Log("Invalid URL: {0}", line);
    }
}




You can remove the variable result from ValidateUrl:

private static bool ValidateUrl(string url)
{
    Uri uriResult;
    return Uri.TryCreate(url, UriKind.Absolute, out uriResult) &amp;&amp;
        (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);
}