None of the social media scripts are best. If you are looking to just start a tiny social networking web site with no originality and no plan to scale or grow it then PHP Dolphin is a great inexpensive start. But if you plan on starting a Social Networking web site in hopes it becomes somewhat popular you're better off developing a custom script rather than using a script that anyone could buy for a couple hundred bucks. I couldn't imagine FaceBook or even a site with <100k members using a script. You would have 1,000 clones over night.
You don't just "script" a proper social networking site. You study graph theory and dynamic programming techniques until you can communicate telepathically with whales and taste colors they don't have names for yet. Then you start coding the first prototypes and blowing VCs for the wheelbarrows full of money you'll need to poach top developers from other companies. Anything else and you're just pissing into the wind.