I am looking at coinmarketcap and i see that indeed the Steem has decreased in size by about $9mln, in the last week.
I think it is possible that this is just the first wave of big posts making big money being powered down and sold. Steem is in active development at the moment and the community growing out of the facility to blog and earn is a factor in what moves people to buy in or cash out.
Once the codebase settles a bit, and a lot more ways are made to add value, the price should start to move towards a steady uptrend. The simple fact that as the sweat-equity contributors on the blog-discussion side goes, are not necessarily adding money in, but we are getting vested tokens back out that take time to cash out, should help people stop and think for a little while 'hmm well I'm not gonna see all of this SP back before 2 years, I might have another look at this, maybe buy some SBD, or even maybe buy some SP and use that to exert my influence as a curator'...
I think coinmarketcap.com marketcap figure might be a bit misleading, but I might be wrong. There is one problem to your last paragraph argument. No social network so far has managed to make people pay for to use it. Donald Trump in his AMA received 600 dollars worth of reddit gold. That is president candidate AMA. That would mean any trending top posts getting 10-15 bucks in steem. Yes. It could go that low too if no incentives for steem buying are available.
The value at coinmarketcap.com are denominated at base in BTC so the value of BTC has an influence on the figure as well. If BTC goes down, so does this number. A more complicated tracking system would be required to see that value in dollars, and besides, Steem can be traded for several other tokens as well, and is, so really, that value should account for multiple pair crosses... The numbers of different tokens going into exchanges is getting dizzying.