There are really three types of giveaways in my mind - and each has a different purpose:
* Thank you give-aways to people who helped you with your book - there are usually less than a dozen of these, often people who you mention in your acknowledgements - but it is a great way to appreciate those who have helped in some way. They become good cheerleaders for you and we all know that personal referral does sell books.
* Review copy give-aways to people who you are asking to do reviews, provide interviews, or write articles about you, your expertise or your book. When these are well-placed and followed up on, they are "gold" to your book sales.
* Contests and other promotional give-aways - these are often in conjunction with an appearance, a radio interview, for raffles to your key audiences, for charity events where your likely readers could be in attendance and so forth. These have mixed results depending upon the other promotion around them and how targeted the audience is for your genre.
I tell my clients to plan about 5% of your total anticipated first years sales to various types of give-aways.