You are only as good as your current product: Facebook's commodity is people, and people are not as loyal as you might think.
Facebook could very well be around a long time, but 'forever and ever and ever'? Smart people have lost a lot of money thinking like that. I certainly wouldn't gamble on it.
Any email provider also has your social graph. The only problem with email is that it is seen as "old" compared to the "new" of Facebook and Twitter. Something else will come along and be the new "new" and that will be the end of Facebook.
They really don't. Not the way Facebook does; not for the post-Millennials. People email bosses and professors and acquaintances; they use Facebook for friends.
Google can easily de-throne facebook from the analytics crown:
1) establish a semi-dominant browser with automatic updates: done (chrome; strong on desktops, very strong on mobile)
2) establish "privacy mode" - disable likes in page, add them as a browser feature. Opt-in of course, but warn users about the privacy implications for letting a "like" button on the page go unquarantined.
3) wait a little and enjoy as Firefox (and Opera and Safari) do the same.
Within a year, facebook analytics stronghold is gone. Google has all that data from google analytics, which is about as prevalent as "likes", but does not look as devious (even though it is).
Facebook's SSO might be here to stay, but I'm sure Google are already working to undermine that as well. And that makes me happy. Not because I think google is less evil than facebook (it is less evil for now, but that could and will change after a couple of bad quarters) -- but because facebook's "like" buttons are a plague upon the web, the functionality of which SHOULD be included in browsers (assuming people and web masters want that, and it appears they do).
(Writing this as a happy RequestPolicy user who is not telling Facebook or Google Analytics anything he does not want them to know, except an IP address; I don't want them to know that, but I do have to tell them).
[1] http://investor.google.com/financial/tables.html -- compare "Revenues" to "Total Advertising Revenues".