I haven't done it, but I did some research...and let me explain how they work.
You see a website saying FREE PRIZES*! You go in and you sign up for one offer, say, a Credit Card.
Then, you get 5 people to sign up for those FREE PRIZES*! program.
Then, and only then, do they send you the free prize.
But, by this time, you gotten 6 people to sign up for a Credit Card. The program has gotten a lot of referral money.
Further, for those 5 people who signed up for the program, each of
them have to recruit 5 new people in order to get the prize. Meaning, you got 125 people now, and each of
them have to recruit 5 more people...etc.
It's a pyramid scheme, bound to collaspe. Eventually, the people on the bottom of the chain would not be able to recruit (but signed up for a credit card), saving the company valuable money in sending out the prize. I do not think the scheme is illegal, just that only people who get in first are most likely to be able to get the 5 people necessary to receive that prize. But the prize isn't free, because you still have to sign up for something, like a Credit Card, and the Company giving out the free prizes get tons of referral fees, some of which is used to pay for the prize, but most of it being kept for profit.
That's the ideal of how it works. "Forgetting" that a person has qualifies after getting 5 people signed up is another way to save some money in sending out a prize, helping the bottom profit line.
EDIT: I got the BBB report for one of the companies making some of these Free Prizes* websites,
Gratis Internet. They are being sued, not for using this scheme, but rather for the allegation that they are selling personal information to third parties. So, there's another way people can make money for giving out "free" stuff.
Gratis Internet has an F from the BBB, by the way.