Custom built from the ground up. The ones you still see around are usually custom built.
I would highly recommend against making your own network though. There are just so many already. It's a pretty foolish thing to attempt unless you have a name already made for you and you know all of the ins and outs of the business. How do you expect to get advertisers to sign big contracts with you when your name is virtually unknown? I don't know, maybe you could pull it off, but it would take a miracle and then some.
IMHO of course...
Cheers,
xxf8xx