Pyramid schemes are illegal (in some countries). The rationale for making them illegal is that they must inevitably fall apart some day, because they're based on the continual recruitment of new members. Recruiters drive the price up (or even at least keep the structure going) by recruiting more participants or salespeople rather than by selling a product.

All communities have a limited number of people, even our planet, which shows hard limits to the ability of a Pyramid or Ponzi scheme to expand.

Ponzi schemes are able to keep going because new participants are constantly brought in, such that any existing participants can withdraw their earnings because the new recruits have put their money in. Thus, participants are promised returns on their 'investment.'