OK
I send it to 100 and get moved to position 2 each of them send it to 100 and I get moved to position 1 Each of them send it to 100 and I get $5000
Already 1,000,000 people have participated. Now lets assume theres a 1% take up (quite high) I'd have to send it to 10,000, as would the sencond and third persons. Thats 1,000,000,000,000 - 1 trillion people, about 150,000 times as many people on earth.
Lets assume a 100% uptake. I would get my $5,000 having started it, but the second person would have to wait for person #3 to send it out, increasing the numbers participating to 100 million. The person at position #3 would have to wait for 10 billion people to participate - everyone on earth and then some.
Even assuming everyone in the world participated, only the first person and the second people (101 people) would get the money, about half the third chain would get the money (maybe 5,000). Thats if everyone in the world (not everyone on the internet, everyone from Joe Bloggs in Arkansas to Lu Chin in the Nepalese foothills) participated.