The social network is adding private messaging capabilities to its Pages feature, but not to all Pages, and not in the form you are thinking of.
For starters, only business pages will have access to it, meaning pages that use the company's advertising feature to attract new users or sell products.
Page owners can reply to comments with private messages
There are a few ways users will be able to distinguish which Page owners have access to this feature.
First off, a "Send Message" button will start to pop up in various ads, which, as you can tell by yourself, will let customers send a PM to the company's support staff, asking for various details about that specific product.
On the other side of the boat, Page owners will be able to tell from which ad the message came, and will also be able to answer with a canned response, if they want to set up such things.
Page owners can also respond with a private message to user comments, this ability being available via a "Message" link under each user comment.
As for the UI, private messages as a page owner look just like the PM interface in regular accounts, so there won't be a need to train PR staff beforehand.