Damnit, looks like I missed this one.
How likely is it that someone who invested in day 6 or 7 will see a profit? Not likely IMO. Games like this reward the early investors. If I'm not one of the people who invested in the first day or two then there's a high chance that I won't get paid. I'll just keep that in mind and wait for the next round to start.
Yes of course , if you invest only in the first day you will be safe. It is so that a ponzi works :
"
A Ponzi scheme is an investment fraud that involves the payment of purported returns to existing investors from funds contributed by new investors. Ponzi scheme organizers often solicit new investors by promising to invest funds in opportunities claimed to generate high returns with little or no risk. In many Ponzi schemes, the fraudsters focus on attracting new money to make promised payments to earlier-stage investors to create the false appearance that investors are profiting from a legitimate business."
Source: http://www.sec.gov/answers/ponzi.htm