There are different degrees of popularity. The amount of people that shared files using FTP and usenet pales in comparison to the amount of users of P2P networks. I think it's fair to say that Napster and then P2P made MP3 ubiquitous.

I think Winamp, which predated Napster by a couple of years, was really the turning point that made MP3 popular. Really MP3 made P2P networks popular, rather than the other way around.

My 4GB hard disk was full of MP3 one or two years before Napster appeared.

It was the same for lots of my friends.