hook_service_links_alter never fires

Maybe I am a little dense, but I have created a custom module in the hope of using hook_service_links_alter to change the link passed to Facebook from a page. In my instance, I want to pass a specific link (not the node link) to Facebook when a user shares. Use case is video so the player appears on FB rather than the node.

However, implementing this fails and I can't see anywhere in service_links that the hook is invoked (e.g. module_invoke or module_invoke_all) - When does this hook fire?

My example is simple for now, just trying to print the links array:

function ishare_service_links_alter(&$links) {
  dpm($links);
}

Am I missing something?