Ok guys! Here's my thoughts on this:
While the architecture provides plenty of places for this to happen, there isn't a straightforward way to do it.
My next reaction is, we can definitely do this with the messages module (comes with Kickstart 2) and some sort of "use rules to send social messages" service ... like this module here: https://drupal.org/project/rpx
Also, there are a number of "share widgets" that accept tokens ... https://drupal.org/project/sharemessage and it would simply be a matter of creating your own custom token based on the current user id.
Upon a bit more inspection, I think the recommended approach would be to use Messages and create a Message Notify service that integrates with your favorite social service (twitter, facebook, linked-in, instagram, etc). If money and time are at stake, then one of the above strategies would be faster and potentially easier, but less flexible.
STEP BY STEP
1) Create a new Message Type called "Social Share"
Make sure to include the tokens you want in your message. If you want "I bought [product abc] at [mywebsite]" then you will need to create a message per product line item type that is in your cart. These tokens can be passed from rules into messages without the need to create a custom token using code.
2) Create a rule that actually creates the message entity of type "Social Share"
The trick is knowing how to create a rule that can loop around the list of line items in a cart, and then how to add a second rule that gets fired from within that loop that actually generates the messages.
3) Make sure you have a way to generate share links/icons
Now that you have messages being generated per customer per product, you have two options. A) You could create a new "Message Notify" service that posts these messages to a service provider (most flexible, also a daunting developer task). B) You could look into one of the many social modules/services out there that let you set up links with a custom title. One such service that we've recommended in the past is "ShareThis" and they have a nice examples page that gives you lots of cut & paste options.
4) Put the share buttons on your checkout completion page.
One of the easiest ways to do this in code is to use the hook_form_alter() function.
<?php
function example_form_commerce_checkout_form_complete_alter(&$form, &$form_state) {
$form['checkout_completion_message']['message'] = '<pre>add button text or rendered message entities wrapped in sharethis links here.</pre>';
}
?>
