To get your feet wet, start with LinkedIn, which is geared toward professionals. Profiles and photos are typically in good taste. Your profile will read like an online résumé, and you can ask and answer business questions, a great way to make contacts.
Facebook requires you to exercise more judgment. Still widely used by college kids to recap last evening's events, it mixes professional and social - for better and worse. Your boss' snapshot may appear on your page next to a questionable photo of a college buddy. (One would hope that that's not a problem once you're past, say, 30.)
You can delete anything inappropriate posted to your page, but that means you'll have to monitor it. You also have control over who can view what information. So become familiar with the site's privacy settings.
Two other notes of caution: First, be wary of the thousands of applications being developed for Facebook. They do neat things like let you set up conference calls and create business cards, but you'll be handing over personal data to developers that aren't screened, warns Jason Alba, who writes about social-networking sites.
Second, Facebook was recently blasted for allowing advertisers to use your purchasing behavior as a way to market to your friends. The company backed off, for now.
The prudent conclusion: Everybody's doing it. Just use your good sense.
Last updated January 17 2008: 5:45 PM ET
