I would not be interested in using or advertising on such a site. I don't think there's enough value for someone to go to your site but average people value their free time like shit so who the hell knows. I would say one larger item randomly given to one person would be better than earning credits. Even someone who is borderline mathematically illiterate will be able to deduce how terrible the dollar/hour of time to credits to gift cards is. You'd have to also implement a way to ensure only active visitors get credit otherwise people will open up your site in a tab and just ignore it which would drive advertisers away.