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Receiving an e-mail invitation to become the friend of a thirty-something year-old man on Facebook.com might at first be a bit unsettling.

Facebook is best known as an online hot spot where high school and college students keep up with their friends and meet new ones.

But the growing ranks of grown-ups who are populating Facebook aren't in the midst of a midlife crisis. Rather, they are using the site to rekindle old contacts, keep in touch with colleagues and even plan business dinners.

Thirty-one-year-old Vladimir Cole signed up three weeks ago after noticing that the Facebook solicitations showing-up in his inbox were from "serious" professionals -- many of whom he met while studying for his master's of business administration at the University of Pennsylvania's Wharton School of Business.

"I always just thought it was just kind of a dating site," said the Seattle-based Microsoft product manager.

Facebook's strategy -- to become a must-have tool for people transitioning from school to work -- is paying off.

Since September, when the social networking site decided to allow people to join who were beyond school-age, Facebook has doubled its Web audience from 7.7 million to 14.1 million, according to Nielsen/NetRating's May report.

By allowing users to pick and choose from a hodge-podge of tools -- such as classified ads to an interactive map displaying the whereabouts of contacts in your network -- Facebook has captured the fancy of the famously fickle tech industry and is working its way into the mainstream.

These do-it-yourself features have made Facebook attractive to all consumers -- not just teenagers, said Joel Adams, a Sewickley-based venture capitalist.

"The most important thing is that you do the work yourself, the Web site doesn't do it for you," Mr. Adams said. "It's a utility."

For years Pittsburgh native Terrence Chay relied on business networking sites such as LinkedIn. Now, Mr. Chay, who lives in Silicon Valley, turns to Facebook to plan social gatherings where business mixes with pleasure.

Mr. Cole said his co-workers post photos from their office nights on the town on Facebook -- as well as keep in touch with business contacts. He even uses the site to keep track of what city his wife, Fiona, a jet-setting consultant, is working in.

"It's kind of funny; she updates Facebook more than she updates me," he said.

Some in the older group are being bullied into giving Facebook a run by their children and their younger Web-addicted colleagues, said Noah Kagan, a former Facebook product developer who now works at another online firm, Mint.com.

"They want to see what all the hype is about," he said.

Industry watchers say many of Facebook's older members are still mainly "lurkers," who like venture capitalist Mr. Adams prefer to sit back to watch the party -- not take part in it.

"I'm just trying to see what makes these things click," Mr. Adams said.