If it's a new person on the site and I've given them some answers and there's been no response I'll come back and acknowledge the fact that they are new to the site and it's usual to say 'thanks' for any help. It also lets the person know that you have seen the answers they've given. This normally elicits a thank you response.
I once read a great letter in the paper on this very topic. A woman had held a shop door open for someone who had just sailed through without a word. So she said 'Excuse me, I didn't hear what you said'. The person turned back and said 'I didn't say anything'. The first lady then said 'Oh, for a minute, I thought I heard you say 'thank you'! The person went bright red and muttered a thank you.