The Sociable plugin does not implement it's URLs correctly for https.
You can fix this by changing the definition of SOCIABLE_HTTP_PATH in the plugin as follows:
define( 'SOCIABLE_HTTP_PATH', plugins_url( '/', __FILE__ ) ); // Plugin folder URL
Using WP_PLUGIN_URL will cause it to always serve files as http, no matter what the settings in WordPress are. Using plugins_url() ensures that a filter is applied when https is in use, and it will dynamically change the URLs accordingly.