When I looked in my friend's cupboard (at her request) and found that her New Age nurse had given her vitamins with 200 times the RDA, I wrote to her internist. My friend thanked me.

I wrote a letter to my sister's therapist when I sensed that my sister has been masking problems that the therapist didn't seem to be catching on to. (I'm paying for the therapy.)

I believe that people pay more attention to a letter than to anything else. But mostly I use e-mail.