[panarky]

It's only cheap if you assume that today's margin and growth rate will continue uninterrupted.

If margins get squeezed, or if growth slows, or both, the valuation is in jeopardy.

The average user spends 43 minutes per day on Facebook. Last quarter, users spent 2 minutes less per day.

What happens if Facebook is perceived as mildly toxic to many users?

Just like people used to drink gallons of carbonated high-fructose corn syrup every week, almost overnight most people decided that's not healthy, and soda sales have declined every year for 12 years straight.

What if something similar happens to Facebook? If a year from now people are spending 25 minutes a day instead of 40, what happens to the margins and growth rate that justify a $500B valuation?