I think it's easy to get used to the speed of having a book available through Amazon KDP. Smashwords makes the book available on their site immediately (faster than Amazon, actually), but it does take a while to get reviewed and then have the e-book distributed to other sites. I usually figure at least a month.

Alessandra, I've been using Amazon KDP Select as a way of offering the first book in a trilogy free. My numbers are not huge, but I did have over 15,000 free downloads during a 24-hour free promo, followed by around 500 paid sales during the following week/week and a half. I have not had consistent results with free promos.

J. Tanner mentioned on another thread that rumor had it Amazon tweaked their algorithims mid-March. I haven't had more than a few thousand downloads from freebies since then.