What is the blogging software to allow millions of users to create their own blogs on a social networking site? Most of the software available is for one user to create his own bloag and post on his site. Is there any blogging software available that can be used by any user to create his/her own blog, post comment on someone else’s blog etc? Just like in MySpace?
I am looking for software that works on windows platform.

No. It’s custom built. Think of the hardware requirements necessary to handle so many people at once. You need multiple servers working together, load balancing, database replication. Similarly, the software needs to be compatible with the optimised hardware, and be very powerful itself. Unless you got money to buy MySpace itself, don’t expect to get your hands on anything like that.