A new feature was rolled out to today. Now whenever you vote a post up in the forums, an entry appears in your feed indicating that you've done so and the other user will receive a PM as well. Unfortunately, it's not very useful. You can't even tell which post was recommended.

If you go into your privacy settings, you can prevent it from appearing in your feed. And from the looks of it, that will also prevent the system from PMing the other user when you do it. There does not seem to be a way to prevent these PMs from being received though, so I might have to stop posting on the forums until such an option becomes available.