This is an interesting question.
The original concept in the 1920's, was that money was only put into the postal mail program by the joining members. If the existing members together, minus the company expenses, were on average taking out less than the pooled fund, which was not making money from invested interests, all would keep going and work very well.
Today, schemes are classified as Ponzi's, in a different way.
Pips was not strictly a Ponzi scheme, since the input to the pool, was not just from the members joining. There were many, potentially good offshoots, which, had they been different, could have added money to the pool but maybe never enough to sustain the system growth for the members, which was becoming vast. This may have been due to lack of business knowledge, since collectively they only lost money (they were the wrong type of businesses), but with much franchising they could have earned considerable money . There were ways of making the system last very much longer, even without the PPP investment, which was not made, but I shall not go into all the detail here.
The present exponential growth of 'Paid To Surf' schemes is phenominal. These schemes will not last for ever and those that pay more than others will fail first. These schemes are 'part Ponzi' because there are many income streams from the services offered by the websites but there are too many services offered by too many 'Paid To Surf' websites to be viable in the long term. They are all doing what Pips should have done and that is to severely limit any one individuals' income in varying ways depending upon the particular system. The basic limit is to limit the number of months before the credit system has to be refreshed, meaning you need to start again. Hence the growth is not exponential.
As long as we eat well, we will get fat!