@lukestokes is challenging Steemsters to succinctly articulate "where does the [Steem] money come from?", and to do so in a way that the skeptical public will understand. But what he's really asking us to explain is "where does Steem's value come from", and that's what I'll attempt to answer here.
Anytime I am explaining something new to people, it's always helpful to start with something they already understand and to analogize from there. Fortunately, almost everyone today already understands social media platforms like Facebook, Twitter, Reddit, etc.. So, let's start there.
Steemit is simply a social media website like Facebook, Reddit, Twitter, etc. However, unlike on those sites, people on Steemit can "pay" to have more influence in how the site is run--for instance, they can pay to have a greater say than others as to which posts go viral or which ones get downvoted to irrelevance.
On Steemit, it's not "one person one vote" like it is on other social media sites. Rather, it's one Steem one vote. So, everyone wants Steem!
Steem is a scarce digital token, like bitcoin, that can't be forged or duplicated. Also like bitcoin, new Steem comes into existence at a fixed rate (in Steem's case, every three seconds or so). Steemit users can get Steem by either purchasing it or earning it. Why do they want it? Again, so that they can have more social influence and a larger voice in how the Steemit site operates. More Steem = more influence.
Users can earn Steem by posting good content that others upvote, or by curating content (upvoting and down voting it) that others post. Posters and curators are awarded a portion of the Steem created during the 30 days after their post in proportion to how many up-votes or down-votes the post gets, and from whom. Up-votes and down-votes from users with more Steem count more than those with less.
So, where does Steem's value come from? It comes from the fact that Steem is scarce and people want it. They want it so that they can have more influence--a larger voice. Because they want it, they are willing to buy it or work for it.
The larger Steemit becomes--that is, the more users it has--the more people each user can influence and therefore the more having such influence (Steem) matters. As Steemit's user base multiplies exponentially, so should the value of each scarce Steem.
Steem has experienced nearly unprecedented user growth since July 4, 2016 when it existed stealth mode. Importantly, users who try it stay around, and they are already spending as much time per day (on average) on the site as users of many other valuable social media platforms that currently have far more content.