... the idea of Facebook or iOS is the same : to be a platform where others will be compelled to build on top you, like M$ of yesteryears. That's the difference between myspace and facebook.
Any app which requires a social component - "dating" being the obvious one (eg: zoosk / datehookup) - will find it easier to re-use facebook's user-base rather then build on a user base from scratch.
The potential for brain dead social gaming is also endless as shown by zynga. If you don't know what zynga is I wouldn't be surprised but it's heading for a 900+ million dollar revenue this year (revenue, not valuation). If it was an isolated website, it would be lucky to reach 1 million in revenues. facebook keeps 30% of the revenue... about fair share, I would say.
I am 35+. Me and my friends use facebook mainly for photosharing more than anything else and probably we are not targets of the social networking websites. Its the younger generation - in colleges/just out - where they are growing up with myspace, twitter, facebook and texting - the fundamental structure of "friend circle" has changed for them... I hardly had any overlaps between my circles.. but it's a common theme for youngsters to have 30-40% overlap between their circles. Why should it result in better valuation for FB ? There is no direct answer here, but it means more power to social networking tools.
The other users of facebook are still evolving.. eg: it's acting as a news aggregator for communities.. eg: you can do a "like" on west minister abbey" or your local pub and you will get any updates on their calendars. You don't have to subscribe to their newsletter and then mark it spam a month later.
In future, you might have to "pay" a dollar for a <like> (implying subscription) on "the Guardian" facebook. So it just becomes another app store like apple's - but even easier to buy.
So yeah, facebook as a business is a little more than news feed from mostly absent friends.
Bottom line: Is facebook valuation at 75 billion too low/too high ???? I don't know - the current $10-20 billions revenue suggests that a $50billion is definitely ON, except by most conservative evluation models designed for classic industries which have linear & predictable growth patterns.
But anything beyond 100bill$ looks high (to me, a commentard). Why, don't I believe in the social networking fairytale ??? Well, I do believe that any fairytale will come to an end/plateau
sooner or later. It did for Google. It will for Facebook.
To end the long rant: let's say we all hate Apple, Facebook & Google for being evil in the ways they do things... take from us without our permission, snoop on us and in general they de-power us in our digital lives.... And our protests will keep them honest. But there is no denying that they are doing their part in shaping how 21st century will be, and for that, <respect>.