Typically this information is not provided by many startups. User count and activities are a direct measure of company value and during the initial stages of companies they will, typically, keep this information close to their chest (unless it's REALLY impressive). The best gauge you can use to find out info about websites and the people behind them would be blogs that focus around that.
As a result, you'd be hardpressed to find significant statistics on the company and it's success. On the other hand, VC funding and amount of money raised is particularly public and sometimes a good indicator of the movers and shakers. Some sites which focus on the public aspects of exciting companies would probably be your best bet. The top ones which come to mind are: