I don't get why this is any different to anything else. What's to stop Microsoft backdooring the next Outlook, MSWord or Windows itself? Why is there so much ZOMG-FUD over this from LinkedIn than there is over anything else?
"Most users" blindly type the same password into Facebook that they do for Twitter, LinkedIn, Gmail, OK Cupid, eBay and PayPal. Any of those services can (and do) get hacked and the password opens all the other services. Should we shut them down too?