Google at least has a business model, I can't say I can actually detect one in the case of facebook.
As for netscape, yes, in part it was a lost (or destroyed, more likely) opportunity, but at the same time even if they had continued to be successful their valuation at the time was right up there with far future science fiction.
You can target ads at "all doctors under 30 in the chicago area that went to a ivy league school". This type of targeting can lead to huge CPMs.
One example of where they could go: they are one cookie away from a significant branding ad platform.
Another idea: advertise on keywords in activity streams. Tie this to the branding ad network above.
Update: Not clear why this is getting downvoted. These are two huge opportunities for Facebook that could easily justify big multiples. And people said the same thing about Google (no serious business model) before it went public.
I always figured they'd go and do a payment system because in facebook the 'graph' of your friends validates you to the point that doing business with someone becomes a lot less risky than on paypal, but for some reason they seem to be holding back on that.
Maybe the zynga deal is a prelude to that.
It would make good sense to do that, facebook is the closest we've got to a verified ID on the net. The issues to contend with would then of course be hacked accounts and such but that's not different from other payment systems.
As for netscape, yes, in part it was a lost (or destroyed, more likely) opportunity, but at the same time even if they had continued to be successful their valuation at the time was right up there with far future science fiction.