Updated in Telligent Evolution platform SDK 5.5.2.

EndpointGETapi.ashx/v2/users/{username}.xml (or .json)
EndpointGETapi.ashx/v2/users/{id}.xml (or .json)
RequestUserShowRequest
Id Optional Either Id or Username is required.
Username Optional Either Id or Username is required.

If you request a user whose username looks like a number (e.g. “12345”), the API will assume it’s a User ID rather than a username. For such scenarios, you should use the Get Users endpoint, specifying the username on the usernames parameter e.g. /api.ashx/v2/users.xml?usernames=12345.

Example (C#)
var webClient = new WebClient();

// replace the "admin" and "Admin's API key" with your valid user and apikey!
// http://telligent.com/community/developers/w/wiki/how-to-generate-an-api-key.aspx
var adminKey = String.Format("{0}:{1}", "Admin's API Key", "admin");
var adminKeyBase64 = Convert.ToBase64String(Encoding.UTF8.GetBytes(adminKey));

webClient.Headers.Add("Rest-User-Token", adminKeyBase64);

var requestUrl = "http://mysite.com/api.ashx/v2/users/2103.xml"; // or .json
var xml = webClient.DownloadString(requestUrl);

Console.WriteLine(xml);
Example Response (XML)
<?xml version="1.0" encoding="utf-8"?>
<Response>
  <Info />
  <Warnings />
  <Errors />
  <User>
    <Id>2103</Id>
    <AvatarUrl>http://mysite.com/utility/anonymous.gif</AvatarUrl>
    <Bio>sample-data</Bio>
    <Birthday>2010-06-24T15:41:41</Birthday>
    <DisplayName>sample-data</DisplayName>
    <Gender>Male</Gender>
    <JoinDate>2010-03-29T16:19:49-04:00</JoinDate>
    <Language>en-US</Language>
    <Location>sample-data</Location>
    <Points>0</Points>
    <PrivateEmail>sample-f78f6@email.com</PrivateEmail>
    <ProfileUrl>http://mysite.com/members/sample_2D00_30dbc/default.aspx</ProfileUrl>
    <PublicEmail>sample-data@email.com</PublicEmail>
    <TimeZone>-5</TimeZone>
    <TimeZoneInfo>Eastern Standard Time;-300;(UTC-05:00) Eastern Time (US &amp; Canada);Eastern Standard Time;Eastern Daylight Time;[01:01:0001;12:31:2006;60;[0;02:00:00;4;1;0;];[0;02:00:00;10;5;0;];][01:01:2007;12:31:9999;60;[0;02:00:00;3;2;0;];[0;02:00:00;11;1;0;];];</TimeZoneInfo>
    <Username>sample-30dbc</Username>
    <WebUrl>sample-data</WebUrl>
    <ProfileFields>
      <AOLIM />
      <Birthday>2010-06-24 15:41:41</Birthday>
      <Blog />
      <Department />
      <Education />
      <Email />
      <Facebook />
      <Gallery />
      <Gender>Male</Gender>
      <GoogleIM />
      <Hobbies />
      <Language>en-US</Language>
      <Location>sample-data</Location>
      <Mobile />
      <Occupation />
      <Office />
      <OfficePhone />
      <PublicEmail>sample-data@email.com</PublicEmail>
      <Title />
      <Twitter />
      <Website>sample-data</Website>
      <WindowsLiveIM />
      <YahooIM />
    </ProfileFields>
  </User>
</Response>
Example Response (JSON)
{
  "User": {
    "AvatarUrl": "http://mysite.com/utility/anonymous.gif",
    "Bio": "sample-data",
    "Birthday": "\/Date(1277408501000-0400)\/",
    "CurrentStatus": null,
    "DisplayName": "sample-data",
    "Gender": 1,
    "JoinDate": "\/Date(1269893989000-0400)\/",
    "Language": "en-US",
    "Location": "sample-data",
    "Points": 0,
    "PrivateEmail": "sample-f78f6@email.com",
    "ProfileUrl": "http://mysite.com/members/sample_2D00_30dbc/default.aspx",
    "PublicEmail": "sample-data@email.com",
    "TimeZone": -5.0,
    "TimeZoneInfo": "Eastern Standard Time;-300;(UTC-05:00) Eastern Time (US & Canada);Eastern Standard Time;Eastern Daylight Time;[01:01:0001;12:31:2006;60;[0;02:00:00;4;1;0;];[0;02:00:00;10;5;0;];][01:01:2007;12:31:9999;60;[0;02:00:00;3;2;0;];[0;02:00:00;11;1;0;];];",
    "Username": "sample-30dbc",
    "WebUrl": "sample-data",
    "ProfileFields": [
      {
        "Label": "AOLIM",
        "Value": ""
      },
      {
        "Label": "Birthday",
        "Value": "2010-06-24 15:41:41"
      },
      {
        "Label": "Blog",
        "Value": ""
      },
      {
        "Label": "Department",
        "Value": ""
      },
      {
        "Label": "Education",
        "Value": ""
      },
      {
        "Label": "Email",
        "Value": ""
      },
      {
        "Label": "Facebook",
        "Value": ""
      },
      {
        "Label": "Gallery",
        "Value": ""
      },
      {
        "Label": "Gender",
        "Value": "Male"
      },
      {
        "Label": "GoogleIM",
        "Value": ""
      },
      {
        "Label": "Hobbies",
        "Value": ""
      },
      {
        "Label": "Language",
        "Value": "en-US"
      },
      {
        "Label": "Location",
        "Value": "sample-data"
      },
      {
        "Label": "Mobile",
        "Value": ""
      },
      {
        "Label": "Occupation",
        "Value": ""
      },
      {
        "Label": "Office",
        "Value": ""
      },
      {
        "Label": "OfficePhone",
        "Value": ""
      },
      {
        "Label": "PublicEmail",
        "Value": "sample-data@email.com"
      },
      {
        "Label": "Title",
        "Value": ""
      },
      {
        "Label": "Twitter",
        "Value": ""
      },
      {
        "Label": "Website",
        "Value": "sample-data"
      },
      {
        "Label": "WindowsLiveIM",
        "Value": ""
      },
      {
        "Label": "YahooIM",
        "Value": ""
      }
    ],
    "Id": 2103
  },
  "Info": [],
  "Warnings": [],
  "Errors": []
}