first make some research that at what source you can get the lots of traffic could be anything its necessary that you stick up with fb or twitter could be any source
With facebook your exposure is limited to your friends and members of your fan page. With twitter you can reach anybody who listens to your hashtag
I haven't used myspace before so i can't say much about it. Regarding FB and Twitter -> FB can be used for Promotion and Analysis as a FB page can help you promote your product, instantly let a lot those people who liked your page know about the updates from your product and get there feedback. Twitter can be used to let a lot of people know about your product, not much market study can be done but it will surely reach a lot of people.
My vote will be completely for Facebook. As of now, I have never received any traffic from twitter. However, Facebook, too you have to do some viral marketing techniques so you get the most out of it
Those three social networking site are all good for the purpose of marketing/advertisement. But you need to find your target audience since facebook has the highest traffic so its a better window for you to advertize your products or services.
I support facebook, as there are lots of ppl discuss about how to make money with facebook, learned a lot on DP forum and hope I can start my Facebook marketing right now