I had a few that just came my way but I think that if you expect to make a go of it selling stock through your own site you need to market yourself. I use my site primarily to help get assignments and it works well for that but again it's just part of the marketing strategy. The site itself brings in some business but you can't expect to just put up a website and sit back and wait for work to come to you.
It is very effective as part of your marketing strategy though, because editors do browse there on their own, but I mostly use it to show people whom I've contacted - potential clients - both assignment and stock - my work.