LinkedIn Inmail has been my go-to. Find a suitably high ranking staff member who is usually insulated from customers, and they're often shocked to hear real customer experiences.

Got Cc'd on an E-mail chain from the assistant to some senior VP at DHL once, where the threat that said SVP wanted updates magically caused a package that support had claimed was already on a freight ship across the Atlantic in the wrong direction back to the sender to turn up in a depot five minutes from my office.