In another discussion it was mentioned that if you get a free copy of a book in a giveaway you are "obligated" to write a review. Now, I've done this, twice on Librarything. Each time I gave 75+ ebook copies of short story collections. This was four months ago. In that time I have had less than ten reviews - combined - posted. Which makes me wonder if it's worth it to go through all the work. True, it doesn't cost anything, but there should still be a return. According to the "obligation" theory I should have 75+ reviews.
How's it worked for you? Do you think it's worth it, all the giveaways and nothing in return?
Unfortunately, that is the problem with giveaways. There truly is no obligation or requirement to review the book. How could you possible enforce it? The only option would be to track the winners and then follow-up for reviews posted by them. What is the "punishment" for not reviewing the book? Banning them from winning future giveaways? There are thousands of giveaways on the web so I doubt it would be much of a deterrent. Ultimately, it comes down to, as you said, what's the ROI?
First, are the ten reviews that you have from people who won the book in a giveaway? Have you confirmed this? If they are then you have to accept that without the giveaways you could potentially have 0 reviews. I would say that the reward is worth the investment. If they are not then you have to break it down farther and consider the alternatives. Only you can decide if the ROI pays off for you.