SteemIt issues new steems every year and rewards curators, participants and others.
A large social network is inherently valuable because ultimately people are the value. Facebook or YouTube or Instagram are inherently valuable social networks because they are too large and have a lot of people engaged. Their monetary model has to do with advertising.
However, SteemIt attracts fiat money in the form of Steems as it gets exchanged on the exchanges. Steems can be converted to SteemDollars or SteemPower and the vice versa.
So, people who add value, i.e. create content, curate content get rewarded with Steem dollars that can be exchanged on the exchange.
Ultimately, networking, engagement, content are the value any social network has to offer.
The larger the SteemIt network will grow, the more valuable it will become.
It is not a ponzi scheme and it is a real social network with real value.