You dont. It would require alot of work, and we dont really need another crappy site like myspace.
Not to mention, if you knew ANY of the languages required to build a site that complex, you wouldnt be asking how to build one.
Myspace, as lame as it is, has a huge backbone of server side software thats all custom made for it. Its not simply coded in Javascript/PHP/HTML, they have a massive library of scripts to run their audio/video streaming, and their databases. Its not all just flv thats inserted, because 99.9% of the stuff is automatically generated/converted server side before going up.
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Heaven has a beer volcano and stripper factory
Hell is the same, but the beer is stale and the strippers have STDs