In the specific instance of a "thanks" message, I'll somewhat regularly send an out of band message to say thanks for helpful replies(or general comments). It doesn't pollute the threads for others, and I'd like to think the added cost of a personal note gives the reader a sense of the value I found in their writing.
I'm yet to have any pushback on sending a thank you note, but who knows maybe it will upset some people in the future ;)
Which - I guess - also deserves a "add some contact details to profile" note.