If someone stops visiting this message board, then obviously s/he does not want to be here. And since most everyone is an adult, I say you respect their decisions.
If I left a board, and someone from it tracked me down, it would feel creepy to me.
There is a reason the person left, and even if you were able to recruit that person back into the fold, s/he will only leave again.
Obviously there would be exceptions to my scenarious. Still, I think it has the potential to do more damage than good.
Besides, it's not like this is church or anything.
Elphaba