PHP was on the rise before FB because it's terribly easy to build useful websites with it. It would be good PR to have a big cool app which was very publicly Perl and had easy plugin authoring in Perl.
Perl however was *not* popular because it was used at Amazon and Yahoo. It was used there because it was so useful. It's still used just about everywhere to some degree for the same reason. Most Perl hackers back in the day were surprised to hear where Perl was used. They just liked using it. And the idea of a language being "famous" is kind of stupid. When TMZ starts covering Perl I'm headed for the mountains to start my Luddite days.