>>It may not be a chain letter, but to some extent it's not much better if people do start spreading it.
>
>I think it is. Any letter that says "forward to all your friends" - or to the top 20, etc., is a chain letter by definition, whether the message be a fake virus warning, or a legitimate request. The point is that (as you mentioned already) the chain letter (or whatever the heck you call it!) starts spreading all over the known universe, and nobody can stop it.

And there's no need to stop it - it stops by itself in a few iterations more, when the pool of candidate participants is exhausted. Since the chain letters (or chain trading, if you ever saw that) relies entirely on its exponential growth, it ceases by itself very soon.

While I was teaching in high school, I once had to explain to my students that understanding how it works involves just simple math (1+2+4+8+16... ) and that within 32 iterations it needs more people than there are on Earth. Five minutes later, a colleague (an engineer) tried to pull me into one of those "you give ten to me and ten to the guy at the top of the list; you then find two guys who will give ten to you each and ten to the guy at the top...".