Such schemes are asshole-creation-machines.
Once you participate you have a vested interest to convince others to join, knowing that they will have to do the same or lose their money.
I strongly recommend you not to participate in such things, because even if you make money, you might end up losing good friends (or at least gain massive bad karma for trying it on random people in the internet).
This is the right way of reasoning in a perspective of development and general diffusion and acceptance of bitcoin
There are people who treat Bitcoin the same way, so you are partially right. Obviously you have a vested interest as well and there are quite some that go way overboard with advertising it.
There is one difference though. Bitcoin could work fine, if the user amount stays the same for a long time (although the current inflation is still a bit too high, so it currently does need a bit of growth).
Ponzis have to grow exponentially forever (which is impossible) or they will collapse.