I agree with your maths. The $4000 was a bit tongue in cheek. In my post of a few days ago I estimated $100 to $200. That was based on steem achieving a certain percentage of a typical diversified digital assets portfolio.
As far as the FB comparison goes, there are 2 factors which affect your $50 + $200 markup calculation.
At 50m users the future growth expectation is higher than FB’s 2bln growth expectation. That justifies high markup.
FB has heavy costs due to its infrastructure. Steemit’s lower cost justify a higher price per user.
In conclusion $50 per steem looks like the baseline, and there will still be plenty of potential.