One e-mail about a new service is a quirk. Two e-mails in a few days is a curiosity. Three e-mails within the space of a week is a bona-fide trend. And when The Wall St. Journal and The New York Times write stories about the trend, you suddenly have a phenomena.So-called business social networks sprung on the scene last year and, judging by personal experience, the desired network effect is beginning to work. People that have joined these private Web sites are inviting their friends to join, who are inviting their friends to join. While my in-box isn't overflowing, I have received enough invites to spark my interest.Unlike general sites like Friendster.com, which bills itself as a way to meet friends and find dates, sites such as\u00a0LinkedIn.com\u00a0and\u00a0Ryze.com\u00a0are targeted at professionals. While the latter two differ this way and that, the general goal is to help you make connections that, as Ryze puts it, will enable you to "grow your business, build your career and life, find a job and make sales."These business social networks are based on a basic pyramid scheme. If everyone that joins invites friends, the numbers get large fast. I was invited by two people to join LinkedIn, and after I filled in my profile (title, industry, areas of specialty and biography), I accepted their invitations to join their networks. They already had contacts so I was instantly two degrees away from 107 people (the site keeps track for you).In fact, the site said I was three degrees away from 1,400 contacts and four degrees from 28,100 contacts But I could only see the list of people known by my two original contacts. And if I was to try to reach out to one of those 107 subscribers, the request would have to be OK'd by my original contact.I found some interesting folks I want to meet, but it also works the other way. I joined LinkedIn at 3 p.m. and by 7 p.m., there was an invitation from a CEO asking if I would join his network. That was compelling evidence that there is some magic here.But the real value comes when you search the whole membership base for specific contacts, whether you're looking for employees or a job. The site returns people you can contact immediately and others that want you to step through the referral process.LinkedIn lets you do this for free today, but apparently will start charging for the service later this year. Ryze charges $10 per month for its most advanced search capabilities.While interesting and potentially useful, the question is whether these sites will be able to make enough money to survive.