There is the "Personal contact form" option that you can enable in My Account>Contact Settings. I'm not so sure if it actually works though, or where the form shows up. It adds the Email icon/button to the users posts. That's the closest thing to a pm system I can find here.

Click the linked-name of the person you want to pm and leave a message in that user's Guest Book.
That message will be sent immediately to that person as email - if they have "Send email notification" checked under "User guestbook" settings in their "My account" setup.

This email is sent to the person whether their "Personal contact form" mail settings, are set or not.

If they don't have either settings set, they'll at least get the message left in their guest book which they may see when they next log in.