First, Metcalfe’s law is a theoretical result, and may define a realistic upper bound on value of communications networks, but probably not a realistic value.

Second, the value attributed to each potential sharer is only a value for the network itself – not for any return on a campaign. E.g. you could use Metcalfe’s law to compare the value of Facebook, Google+, or Twitter, but not to value a page that’s shared on them.

When it comes to the chart there are more errors:

The text says that cost is a constant, but draws a linear line for it. It also says that value grows exponentially, but plots a quadratic curve.

For an asset truly “going viral”, the cost is a constant (i.e. flat horizontal line), where the value is linear in N (i.e. a straight line).

In fact, Metcalfe’s law does not apply to virals at all as in my explanation above.

Metcalfe’s law is designed to be applied to devices attached to physical networks which can communicate – It’s highly doubted (including by Metcalfe) if it can be applied to social networks at all.