The ads do really help to cover some of the costs of running the site.
I have thought about offering an ad-free version on a contribution / membership basis and also maybe tying this thing some sort of 'verified member' scheme (so other people can chose how much trust they put in them - it's harder to have fake identities where a payment has been involved). I've not really got any firm plans though yet.
The main aim is to keep the site free and available for those who need it.