In your Po.st dashboard, you may see on of your top sharing services is 'Copy text'.
When a user copies text or images from your site this will trigger a share event. When the user then pastes that content into an email, IM, or other direct sharing method it will include a link back to your site with a unique tracking parameter added. Any user who sees this content and clicks the link will be taken back to your site and a click event will be logged.
Copy and paste sharing is one of the most popular ways of sharing on the web. Click here for an article describing the popularity of copy paste.
If you'd like to disable Copy text tracking you can do so within your Po.st dashboard. Click on the Social Tools tab and then update the settings for your Po.st Lite tool.