Is WebClient.DownloadData acceptable for getting data from a website?
Here is my cleanup of the User class. Note that there is a world of a difference between the .Count and the .Capacity properties of a list. Look them up! Your CheckUsername and CheckPassword methods were broken in more than one way.

namespace Foo
{
    using System;
    using System.Collections.Generic;
    using System.Diagnostics.Contracts;
    using System.IO;
    using System.Linq;

    public class User
    {
        private static readonly string filePath = "../users-information.txt"; // Or not readonly if you wish to set it at run time.

        public User(IList&lt;string&gt; tokens)
        {
            Contract.Requires(tokens.Count == 9, "Expected exactly 8 tokens but got " + tokens.Count + " of them.");
            int age = Convert.ToInt32(tokens[7]);
            DateTime dateCreated = Convert.ToDateTime(tokens[8]);

            this.Init(
                userName : tokens[0],
                password: tokens[1],
                email: tokens[2],
                facebook: tokens[3],
                twitter: tokens[4],
                firstName: tokens[5],
                lastName: tokens[6],
                age: age,
                dateCreated: dateCreated);
        }

        public User(
            string username,
            string password,
            string email,
            string facebook,
            string twitter,
            string firstname,
            string lastname,
            int age,
            DateTime dateCreated)
        {
            this.Init(username, password, email, facebook, twitter, firstname, lastname, age, dateCreated);
        }

        public string UserName
        {
            get;
            private set;
        }

        public string PassWord
        {
            get;
            private set;
        }

        public string Email
        {
            get;
            private set;
        }

        public string FirstName
        {
            get;
            private set;
        }

        public string LastName
        {
            get;
            private set;
        }

        public string Twitter
        {
            get;
            private set;
        }

        public string FaceBook
        {
            get;
            private set;
        }

        public int Age
        {
            get;
            private set;
        }

        public DateTime DateCreated
        {
            get;
            private set;
        }

        private void Init(
            string userName,
            string password,
            string email,
            string facebook,
            string twitter,
            string firstName,
            string lastName,
            int age,
            DateTime dateCreated)
        {
            this.UserName = userName;
            this.PassWord = password;
            this.Email = email;
            this.FirstName = firstName;
            this.LastName = lastName;
            this.Age = age;
            this.DateCreated = dateCreated;
            this.Twitter = twitter;
            this.FaceBook = facebook;

            this.AddToTextFile();
        }

        /***** These methods are not great:
        A) It is not safe to send plain-text passwords over the wire as well as well as store them as such. You would want to compare a very well crafted and tested "hash code"/checksum.
        B) This has O(n) run time. Dictionary or a set would be a better data structure. You probably want a dictionary that maps a user name to a pair `[userName, hash]` and a dictionary that maps `hash` to the same pair. Maybe you will need a database, but a linear list is not as great.

        // Unused?
        public static bool CheckUsername(string username, IList&lt;User&gt; users)
        {
            return users.Any(user =&gt; user.UserName.Equals(username, StringComparison.Ordinal)); // Case-sensitive?
        }

        // Unused? Not the best method name - CheckCredentials perhaps?
        public static bool CheckPassword(string username, string password, List&lt;User&gt; users)
        {
            return users.Any(user =&gt; user.UserName.Equals(username, StringComparison.Ordinal)  // Case-sensitive?
                &amp;&amp; user.PassWord.Equals(password, StringComparison.Ordinal));
        }
        */

        private void AddToTextFile()
        {
            using (var sw = new StreamWriter(filePath))
            {
                sw.WriteLine(
                    "{0} {1} {2} {3} {4} {5} {6} {7} {8}",
                    this.UserName,
                    this.PassWord,
                    this.FirstName,
                    this.LastName,
                    this.Email,
                    this.FaceBook,
                    this.Twitter,
                    this.Age,
                    this.DateCreated);
            }
        }
    }
}