I don't think it matters that much what is said, as long as there is a welcoming response and that it happens promptly.
It must be awful for someone new to the site to have their first post viewed 20 times and not have any feedback.
Yes, I understand that a great many members rarely post anything and I am not suggesting that anyone should feel pressured to participate. However if you are reading this and feel 'I dont think I could, I don't want to muck it up' relax. Just say hi, I remember feeling like you do and leave it at that. We are all in this together, after all.