Isn't the problem that no-one is currently qualified to comment on the long term potential growth or sustainability of social media? It simply hasn't been around long enough to make any comparisons with the past useful.

You're right, that there isn't much data, but the fact that user growth seems to have at least stalled (if not started to decrease) in the richest markets in which Facebook has a presence has got to be a worry.

Assuming that the yearly value of a user is roughly proportional to national GDP per capita, Facebook needs to gain 40 users in India to make up for every one it loses in the US.

Appeal to authority is a logical fallacy.

Asking the dude who stayed at a Holiday Inn how to shutdown the nuclear plant will fail 99.999% of the time.

His flaky opinion piece seems to have done a reasonable job of starting a conversation.