I think your site is neat. StumbleUpon users, in my experience, tend to show a high bounce rate (often not waiting long enough for a site to load), so they are not a good test case.

If you're still looking to advertise, Adwords might be a better bet, though your niche, as you know, is extremely competitive.

Aside from that, I have a couple of comments on the UI:

1. After clicking a price tag, you get an overlay which looks to be draggable (hand icon) but is not. It also exhibits a disconcerting zoom in/out effect when you zoom the map with the overlay visible.

2. Personally I think the logo wouldn't be totally out of place on a tub of margarine. No offense intended. Objectively, it doesn't match the colour scheme of the site.