Editor’s note: This is John Reinan’s weekly marketing column for MinnPost.com. To see the original, go to http://bit.ly/aC2QQx.

We don’t get any literally seismic events in Minnesota, but the virtual ground shook here and everywhere last month as Facebook overtook Google as the most visited site on the Web.

According to Hitwise, a prominent tracking service, Facebook accounted for about 7.07 percent of all Internet visits in mid-March, compared to 7.03 percent for Google. That translates into about 150 million unique U.S. visitors for each site, give or take.

But here’s the real news nugget: According to the Nielsen Co., the average Google visitor spends about 1 hour and 45 minutes on the site per month. Meanwhile, the average Facebook user spends about 6 hours and 30 minutes on the site monthly.

So, not only has Facebook caught up to Google in raw traffic, but its users are almost four times as engaged.

The growth of Facebook is a vivid demonstration of how quickly things can change in the online world. It’s easy to forget that Facebook in its current form didn’t even exist four years ago. Four years ago! Facebook began as a service aimed at college students, and until September 2006, you couldn’t open a Facebook account unless your e-mail address ended in .edu.

Facebook represents a high-tech incarnation of one of marketing’s most valued categories: word of mouth endorsement. Twitter also falls into this category, along with other social media platforms such as Yelp, Foursquare– there are hundreds if not thousands, with new versions popping up daily.

It’s a marketer’s dream: practically the entire U.S. population, online and engaged, ready to spread the word about a product or service.

But like any marketing effort, it only works if the product is good. You can’t fool people into touting something they don’t like. But if someone likes your store, or restaurant, or cereal, they can start an online ripple that can spread to thousands or even millions of people within days.

A Twitter user with thousands of followers is potentially a one-person news service for the things he likes. Same with Facebook– the person who spends hours each week on Facebook updating her friends is likely to tell them about her great experiences with a company or a product.

Identifying and engaging with these powerful social media users is the biggest new challenge in the marketing world. And since someone right now is probably dreaming up the next Facebook, it’s a challenge that will never grow old.