Ryan Slack was waiting in the coat-check line last year following a networking event — a real estate mixer sponsored by the Web site PropertyShark.com — when he overheard a conversation between two men ahead of him.

The men, who were each holding a stack of business cards, said the party was the best networking event they’d ever been to, Mr. Slack, who at the time was PropertyShark’s chief executive officer, said.

Mr. Slack was pleased with their reaction, but nonetheless wondered how to translate the success of the event — which brought together brokers, architects, lenders, designers, and other professionals from diverse areas of the field — into something more tangible.

“What can you do with 20 business cards?” the 36-year-old said in a recent interview. “E-mail everyone individually? How do you follow up?”

Next week, Mr. Slack will present his answer: a new social networking site with which he aims to connect real estate professionals of all stripes, MyDealBook.com. Mr. Slack, who describes the site as a cross between LinkedIn and Facebook, said he believes it will make it easier for new people to break into the tight-knit, relationship-based world of real estate, a contribution he said is especially important in the face of a slowing market.

“With the real estate downturn, people have a greater need for self-marketing,” Mr. Slack said. As competition increases for each deal, the new site will help “increase their visibility within the community,” he said.

Mr. Slack seems to know what he’s talking about. In four years as CEO at PropertyShark — a site that gives users access to property record information, including building details, previous owners, sales prices, and comparable sales — he increased the number of registered users to more than 300,000 from 4,000, and pushed its revenues into the millions. In 2007, Crain’s New York Business included him on its list of “40 under 40.”

Mr. Slack left PropertyShark in April to found my MyDealBook, which is partnered with PropertyShark but is a separate business.

As with other social networking sites, MyDealBook users create profiles, then invite others to become part of their networks. The user-policed site, which allows members to flag questionable content, also shows sale listings and deal information and allows users to form groups.

Essentially, what MyDealBook does is “help you build your team,” Mr. Slack said. After people meet, they can follow up by connecting on MyDealBook. From then on, “you’re one step closer to being able to call them to work with them,” he said. “They’re not a business card at the back of a drawer.” That’s particularly important for real estate newcomers, who lack the contacts more established professionals rely on to execute deals quickly and efficiently.

A senior vice president and top-selling broker at Brown Harris Stevens, Elaine Clayman, said a site like MyDealBook would have come in handy when she first started in the business years ago.

“I used to say that I changed mortgage brokers more often than I changed shoes,” she said. “It takes a while to find people you trust. It’s very important to get people who are not going to drop the ball.”

While she now has a firmly established network of contacts, MyDealBook “could be very good for new people in the industry who want to know who’s out there,” she said.

Other real estate sites — such as ActiveRain and Trulia Voices — are aimed primarily at residential brokers, Mr. Slack said, while MyDealBook is one of the few to focus on both commercial and residential real estate.

An associate director at the commercial real estate firm Newmark Knight Frank, Daniel Hassett, said the site could fill a niche not covered by services such as CoStar, the subscription-based Web site commonly used by office leasing brokers that offers only limited information about other industry professionals.

“Real estate is so relationship-driven,” Mr. Hassett said, adding that he could foresee using the site, for example, to find a lawyer with specific specialties. “You could find an attorney who’s really familiar with a landlord’s lease. It would be more efficient.”

By helping real estate professionals find colleagues and investors more quickly and efficiently, Mr. Slack said the site could also help strengthen the market by increasing the possible number of deals.

The director of housing economy at Moody’s Economy.com, Celia Chen, said that’s a possibility. “In a period when a housing activity is weak, any tool that realtors have and deal-makers have to network is a useful one,” she said.

As an Internet CEO who has already spearheaded one company, Mr. Slack said the site has to be high-quality if it is to build a following among respected professionals. “I’m very concerned about a small number of people doing bad things on the site,” he said. “We’re trying to close all the loopholes.”

The site has a number of features to prevent MySpace-style spam, he said, such as a limit to the number of e-mail messages that can be sent at one time. Certain features will be available to users only after they’ve been on the site for a certain amount of time and have built up a network of considerable size.

“We’re putting a lot of controls on who can contact who,” he said.

The site borrows a few elements from the Facebook model, such as a newsfeed-style feature that alerts users to each other’s activities. But users cannot write public comments on each other’s profiles, a nod to what Mr. Slack views as the future of social networking. While sites such as Facebook and MySpace bring together contacts from all walks of life, MyDealBook will be open only to business relationships.

“Business, friends, and dating are three different things,” Mr. Slack said, adding that in the future, “we’re going to see more specialized uses for social networking.”

Mr. Hassett, who is a member of LinkedIn but “barely uses it,” said the key to MyDealBook’s success will be the number of productive partnerships it forges.

“I think it’s a good idea,” he said. “The question is, will people spend the time to populate it?”