While I do keep up with myspace, facebook, and a couple others, I must say my very favorite is Okcupid.com
I've had an amazing amount of success on that site and attribute it to a few factors: It's easy to use, easy to find online people in your area, easy to flesh out your profile, and since profiles gives a lot of personality (between the free response, tests, and other various gimmicks) it makes it really easy to open a girl by commenting and/or negging on something you find on her profile.
Also in my experience myspace and facebook are primarily social networking/keep up with old and current friend sites as opposed to a site like okcupid which is geared towards dating/meeting new people.
But, everyone has their own experiences...so best of luck to all using any and all sites
