For quite a few years people used the Internet Explorer icon on pamphlets and posters as an icon representing The Internet. It seems people are now starting to use Facebook's icon for that.
This was user testing with groups of young people in Tower Hamlets, London in 2010.
Aside from testing the UX spec we talked more widely around discovery and every group indicated that they began most tasks with a search on YouTube. Music, games, movies, homework research: all started visually on YT.
Facebook came up because the majority of these kids didn't have email addresses, instead doing all their email-like conversations on Facebook messaging and their IM on Blackberry BBM.
This really underlines why buying YouTube was a smart move for Google.
Walled gardens for the less technical savvy have been the norm. Outside of the hacker/coder/geek culture, has it ever really been different? Prodigy -> AOL -> MySpace -> Facebook
It's only a matter of time when something else will replace Facebook as "the Internet". These things work in cycles.