Here’s a good take on why MySpace works. The same thing applies to all the other emerging stand-alone tools mentioned below.
O’Reilly Radar > “remove the web developer and the web gets developed”
In short, it seems that while many people thought of “blogging” as a thing that was understood and more or less set, MySpace asked themselves what kind of job the software did for most users, and figured out how to do that job better. Myspace is actually not as good as any of the standalone apps — blogging, photo sharing, IM, or email — but it puts them all together into an easy to use package that is not only more comprehensive but also easier to get started with.