This is not just a net thing, by the way, although the net certainly spawned it. It's the hottest new trend in the world of marketing. Using ordinary people to shill for products. Often the people are not paid for their "work" or are given some kind of token or pittance. Many people do it just for the thrill of it, or because it gives them some sense of power.
It happened when some marketing types observed that certain people in social networking environments (like Friendster) had more connections than others. These tended to be people who sat at the edges of un-related networks of friends. They also tended to be people whom others saw as trendsetters, and whom others looked to for advice. They were the "hubs" that connected many people.
The idea dawned: if we could get these people to push our product, it would be as powerful as having a friend recommend it, but even greater, because these people have SO MANY friends.
Now there are agencies that specialize in this kind of marketing by seeking out such individuals and offering them small incentives to push products. These stealth marketers then go around chatting up the products at family gatherings, club meetings, parties, etc. I'd guess there's an online component as well. It's surprising the number of people who have jumped at the offer. (Or maybe not surprising when you consider that the type of person who has such a large social network probably has many superficial, transient friendships rather than a few deep, abiding ones.)
There was an article in the NY Times about this a while ago. It's behind the "Times Select" wall now, but if you have access you can read it:
The Hidden (In Plain Sight) PersuadersMany of the takers saw no problem with this relationship. Some considered it innocuous, if the product was something they liked, used and would recommend anyway. And some genuinely liked the idea of secretly influencing their friends to do things for their own benefit. I guess the spirit of Machiavelli is alive and well, even if they don't read him anymore.