For me, the valuation of FB is closer to a TV channel with an audience of c. 500m people for 30mins every day of the week. And one which shows ads to people based on their own interests. What's that worth? I'm not sure.

The comparison with a phone network isn't quite right because I can't see FB ever being able to charge for service.

But I agree with your point. I think Linkedin is far more likely to be wiped out than Facebook, but the market thinks different.