Many users have been stung over the years by changing ISP and losing their email address. Or by not changing ISP, but their ISP changing their name and their email address going out the window.
I think most people have a hard time seeing Google or Gmail disappearing from the face of the internet. And for those that are concerned, they can use their own domain on Gmail.
However users may be less certain of Facebook's long term position. After all, look at where ICQ, MySpace, LiveJournal and the others are today. Maybe this is just a recognition by Facebook's own user base that they're happy to stick around for so long as Facebook is where things are happening, but that they have no great ties to the site and don't necessarily want to create them either.