Firstly, great work in your post. Has a lot of us thinking.
I'll try have a go at valuing Steem. If you identify the valuations of the incumbents in the space Steem.it wants to disrupt, I think of Reddit and You Tube (at this stage I dont see it becoming a Facebook and it probably shouldn't try to? It could end up doing too much etc..).
Current valuations for (from various sources):
Reddit: ~$1.8 Billion
YouTube: ~$75 Billion
Let's say Steem could replace these two (unlikely but let's just say it does).
That's a total valuation of $76.8 Billion.
76,800,000,000/ 263,516,864 (total steem) = $291.4 (assuming all users move over to steem)
This is best case scenario for overtaking incumbents at current valuations. However, Steem could prove to be more valuable to people so it may overcome this.
If Steem.it just replaces Reddit we could see a price of Steem at ~$6.83 (assuming all users move over to steem).
If Steem.it just replaces YouTube we could see ~$284 (assuming all users move over to Steem (or Dtube rather)).
My pick would be that Steem gets to somewhere in the $100-200 range in the next 5-10 years especially if they focus on developing/marketing Dtube.
A $1500 price for Steem would be a $395.3B valuation. Which could actually happen if Steem was on a global scale but keep in mind this is half the current valuation of Google. Facebook has a $500B valuation and it has a fairly hefty user base (2 billion). This is pretty ambitious. You would need approx. 1.6 billion users on Steem to be around a $400B valuation based on Facebook's data.