I recently sent an email to my college roommate, regretting that we hadn’t been in touch and suggesting that we talk on the phone soon.

“Why?” he emailed back. “My whole life is on Facebook.”

Social networking among American adults has nearly doubled in less than three years, and Facebook is cornering the market.

According to the Pew Internet and American Life Project, nearly half of all U.S. adults — 47 percent — are engaged in social networking on at least one platform. That’s up from the 26 percent of adults who used social networks in 2008.

A stunning 92 percent of adults who use social networks are on Facebook; MySpace trails with 29 percent, while LinkedIn and Twitter follow with 18 percent and 13 percent, respectively.

Another stunning fact: of those Facebook users, 52 percent say they’re on it every day.

Enough with numbers. In plain English, what this means is that Facebook is on its way to becoming the most used, most accepted form of personal communication in our society. You don’t need to call, write or email; just check Facebook and see what’s happening in the life of anyone you care about.

View their photos, click on the articles they recommend, see what they think of the latest “Glee” episode — it’s all there, often in real time.

I went to Tilia with some neighbors last week. A couple who usually joins us was on vacation, and we commented that they were missing the fun. One member of our party instantly took out his phone, snapped a photo and posted it to Facebook with a note to our absent friends. The whole thing took less than a minute.

Marketers have always maintained that word of mouth is the best form of advertising. Facebook and cell phones — along with other interactive platforms like Yelp and YouTube — allow for instant word of mouth that can quickly spread to hundreds, even thousands of trusted contacts.

A mention on Facebook doesn’t yet have the power of an ad in the glory days of TV, when there were only three networks and marketers could easily reach a third of all Americans with any message they cared to send.

But it’s growing, and it’s powerful. It’s easy to forget that Facebook has been open to the public for less than five years.

It’s hard to imagine what will happen in the next five.

This is John Reinan’s weekly marketing column on MinnPost.com.