Last year, Facebook itself began facilitating professional networking with the introduction of “pages” (profiles for businesses or professionals that other Facebook users can become “fans” of and receive updates about).

“When somebody joins your group on Facebook, they’re much more likely to be receptive to your message,” said Dustin Luther of Calabasas, Calif., who leads real estate seminars for sales agents and is the founder of the popular Seattle blog RainCityGuide.com. “My last seminar, a group of us all went out to dinner and we were able to post photos and videos. It kind of keeps everybody engaged in what you’re doing in an informal way.”

Yes, plenty of people think social-networking sites are incompatible with the formality of the workplace. Those who professionally network on Facebook and MySpace respond that all business is personal, and that it’s the informality of the social sites that makes them useful.

Leila Hebden, who said that when she began her career in music management she could not have lived without networking on MySpace, likes the site’s casualness and lack of hierarchy. Music industry executives have long seen the advantage of scouring social-networking sites to find new talent, make professional connections and promote their work. One could argue that most other industries are simply coming late to the party.

Still, Ms. Hebden said, if she were in serious economic trouble, MySpace would not be a life raft. “In the face of ‘upheaval,’ ” she said in an e-mail message, “I still think 9 out of 10 people would drag their contacts list out of their Outlook work e-mail than turn to their MySpace community.”

Or not. When Ryan Kuder, 34, of San Jose, Calif., was among the hundreds of people laid off from Yahoo in February, he gave a real-time play-by-play of what it was like by using Twitter, a service that enables subscribers to electronically (and almost instantaneously) broadcast what they’re doing. “The H.R. guy is on his way over to confiscate my laptop,” Mr. Kuder wrote. To network with other Yahoos, as employees call themselves, he also created a Facebook group: “I Worked at Yahoo! Until Today.”