Great question. I've tried with my products that have a cost involved and have sold nothing, but like "GetThin.." I haven't been very aggressive about it. Just reading this made me think, "Gee, I would put a link to our Halloween software for kids." (Wish we could be promotional HERE! - ugh)
I can't tell whether my app or link to my ebook gets downloaded from twitter positings because they are free and I can't tell what comes from where. Gotta go and post a link now. THANKS for asking!