For what purpose? Keeping in touch with friends? Facebook. Keeping up with news/events? Twitter using tweetdeck. Business networking? LinkedIn.
My Space and Facebook are the top most social networking sites used for both business and personal promotions.
Facebook hands down.. assuming you can mind-filter the flying sheeps and all fingers poking every inch of your body.
I think mypage5 is the best social network because it can able to earn money through it, so it is good for earning money...