Online social networks are all the rage today. From Facebook to LinkedIn to MySpace to Twitter -- and countless other sites with a social networking hook -- more and more people are meeting their friends and family at these websites. And, not surprisingly, so too are the many businesses who hope to reach these people through targeted advertising. Perhaps YOUR business.

But are you really reaching the people you want to reach with your messages? Are you sure?

A recent study of more than 200,000 U.S. social media users by the Nielsen Claritas market research firm found some very interesting demographic differences between users of the most popular social networking sites. Specifically, more than 58 percent of LinkedIn users have annual household incomes above $75,000, as do more than 43 percent of Twitter users and 42 percent of Facebook users. On the other hand, only approximately 32 percent of MySpace users had annual household incomes above $75,000. In fact, about 37 percent of MySpace users had annual household incomes less than $50,000 while only about 17 percent of LinkedIn users (and about 28 percent of both Twitter and Facebook users) had annual household incomes of less than $50,000.

According to Mike Mancini -- vice president of data product management for Nielsen -- MySpace users are "in middle-class, blue-collar neighborhoods," while "Facebook [use] goes off the charts in the upscale suburbs."

So while the jury is still out on the effectiveness of advertising your business via social networking sites, if you're going to spend money targeting their users, at least be sure you're aiming at the right target.