Not a Ponzi scheme, as they companies the shares represent exist, and have value (in most cases).
Until my former company finally went public, I didn't really understand how the issuance of shares occurs. In the case of my former company:
1) My former company sold the shares in one vast batch (~$3B) to a bank, which paid for them a fixed rate per share, before the IPO.
2) On IPO day, the shares were sold by the bank on the stock market, at whatever market value the market decided was right. Some shares went to mutual funds, and many to hedge fund investors.
3). Share prices now fluctuate with earnings estimates, world news, and in reverse correlation with the S&P500 (they typically go down when the S&P goes up, and vice versa). I attribute this to hedge funds looking to beat the market trends.
4) Share prices also go down when my former company sells additional shares annually from their ESOP (Employee Stock Ownership Plan) to pay out retirees. Most financial pundits don't understand that the ESOP value essentially hinders the share price, and annually, causes it to drop.
5) No dividends are anticipated, as the company issues new shares annually to current employees, using profits to shelter them from taxes. The CFO and CEO are heavily invested in the company, and they lose $ if their company's value tanks. And they make personal $ if the value goes up.
6) Many of my former coworkers will likely be ESOP millionaires by the time they retire, as long as they've become fully vested in the ESOP, work there for 15-30+ years, and make it to retirement age (62 for ESOP distribution).