Xing is Europe's leading Business Network with more than 2 million members. Recently, Xing announced that they would come up with an API later this year to get access to the network. As far as I know, Xing was developed by ePublica using Perl and MySQL.
Having an API is essential in these mashup days. I was invited to the private alpha test and implemented a reference implementation of an API client via PHP5 which behaves like SOAPClient (but uses ReST as the transport mechanism) and overloads the methods that are available.
Here's an example of how to call it currently (API is subject to change as it is in an alpha state):
$client = new XingClient(XING_DEVELOPERKEY, XING_OWNUSER);
// find my contacts and return some object properties
$result = $client->userContacts(array('id' => XING_OWNUSER,
'attributes' => 'display_name,company_name,current_company,business_address,photo_urls'));
Currently, for security and data privacy reasons (I have to say that Xing has always been an example of how to protect their user's data/privacy), the API lets me only access my direct business contacts and furthermore scrambles the personal data of the contacts.
In the last few days I had a bit time during a cold and wrote my own mashup - marking my Xing contacts on a Google Map. Click on continue to see some screenshots and a bit of explanation ...