The charts below display the trends of Smashwords sales (red lines) compared to the number of Smashwords sample downloads (blue lines) for my three major works.
One thing I notice is that Smashwords users tend to do a lot of downloading, but not a lot of buying. My free download "Entrapment" (only available through Smashwords) just recently passed the 1000-download mark. My hope had been that the free sample would drive users to buy a copy of my other works. That really hasn't panned out as I'd hoped.
Now, for a comparison of sales via Smashwords to the sales made through B&N and Kobo:
For Seeker, the only "sales" made via Smashwords were free giveaways using coupon codes. Kobo sold half as many (but as full price), while no sales were made over at B&N.
For The Cure, the only "sales" made via Smashwords were free givewaways. Kobo didn't sell a copy, but B&N still made seven times as many sales over Smashwords, and all of them were for money.
For Death Brings Victory, Smashwords was the only spot that sold copies, and 80% of the sales were for full price. I will point out that it took several months for this title to show up over on B&N, which may be skewing the numbers a bit.
That said, it certainly does look like most Smashwords users are out for free stuff only. So, perhaps the thing to do would be to use them only for the sites where I can't distribute books myself, but go directly through Kobo and B&N myself for each title. In fact, I've recently posted up Death Brings Victory over at B&N using the PubIt feature...so we'll see how it does in a head-to-head competition. Someday soon, maybe I'll do the same thing over at Kobo.