This is true. And facebook. I'm always forgetting about those.
Aren't those generally places where people have to already be following you? I'm not real big on social media, so I'm not sure what kind of exposure it offers beyond getting the word out to people who are already in the loop. I guess maybe "retweets" and the like could help?
What I wonder is what is involved in getting on all of the various sites out that which host or link to sample galleries (I know your sample pages are linked to from some of them), and whether maybe the sale was mentioned on the sample gallery pages that people might be getting linked to from some of those sites.