The rewards paid to upvoted content creators (like your post) are paid using money (similar to bitcoins) that gets created out of thin air automatically every once in a while.

Part of this money that keeps getting created out of thin air automatically goes to people invested in the platform (people who have high Steem Power). You can buy this Steem Power with USD via bitcoin, and it's a lot like stock - gets traded on a special exchange within Steem. It can go up or down in price depending on supply and demand. But basically if you invest now, you reap rewards the more that new money gets created out of thin air over time, which is connected to Steem's new users posting new stuff and connected to people upvoting the good stuff.

The posting rewards bring in people who want to post stuff, as well as people who want to invest in the platform using bitcoin or US dollars, since they feel this thing is going to absolutely explode & are looking forward to earning a lot as those rewards get redistributed over time.

So basically, it's like a self-watering cactus plant. It keeps growing and growing, and if it ever grows too fast for its own good, the free market can regulate the price of Steem on the exchange, so that it slows down for a bit before picking up again.

Of course there's a lot of stuff that could happen that might cause it to fail, nobody really knows yet what will happen. But that's how it is sustaining itself.