It looks like the main activity is getting other members to sign up. That’s where the bulk of the revenue comes in. And, there’s a multi-level part of people string too. If you sign someone up and they sign someone up, you get revenue from that. That smells of scam. Most of the bulk of the stuff I’ve seen on the Internet is people asking others to sign up under their account.
However, there is free membership, so you can join without paying a cent (although I am sure you’ll be pressured to upgrade your account to entrepreneur status). Plus, there is suppose to be some income from getting and reading spam. I guess the idea is that a company pays People String for the privilege of mailing their members, and you get a cut. I can’t imagine that making much.
What I would tell your sons is what is the purpose of People String. If the sole purpose is to make money, and the bulk of the money comes from signing up others, then it is a hollow organization that is bound to collapse. A company cannot exist for long in order to sign up members hoping to make money.