Are you a young, or even not-so-young, New York lawyer? If so, the odds are good that you’re on Facebook. If not, you’ve probably ignored a dozen invitations from friends urging you to join.

Trust me: You will succumb. Maybe you resisted getting a cellphone 10 years ago, or a Blackberry five years ago, holding out while your friends submitted themselves to the electronic leash. But eventually you gave in—because everyone else was doing it. Lawyers are like high schoolers with six-figure allowances.

Among associates at large law firms, Facebook passed the tipping point sometime over the summer. Since the site opened to the public last year, adults everywhere have been joining—there are 40 million people already on Facebook, and about a million more every week. But lawyers seem to be particularly enamored of it (as is Microsoft, which is reportedly considering an investment that would value Facebook at as much as $10 billion).

It’s an expensive love affair. A British study found that visiting social networking sites like Facebook during working hours costs U.K. firms over $260 million a day. The equivalent number for the United States is surely a multiple of that. Next year, the AmLaw 200 law firms are expected to hire 10,000 new associates. Let’s estimate, conservatively, that half of them spend one billable hour a week on Facebook. If we assume (again conservatively) an average hourly billing rate of $200, that comes to about $50 million a year in lost billable hours—and partner profits. Fifty million bucks will buy you a lot of Hermès ties.

And good luck to any firms who think they can ban Facebook. Allen & Overy, in London, tried that, but backed down after a mini-revolt from employees.

How did this happen? Lawyers—big-firm lawyers in particular—are notoriously cautious about what they share with the Outside World. Why are so many of them jumping at the chance to bare their souls online?

One of the main reasons may be an addictive little feature called the “status update,” in which users can provide a pithy description of how they’re feeling or what they’re up to at any given moment.

Big Law associates love using the status update feature, especially to complain to other lawyers about their miserable lot. It may also be that these lawyers, whose days are divided into six-minute increments which must be accounted for, yearn to give any status update that is more alluring than, say, “reviewing lease agreements.”

Here are a few status updates from actual Big Law lawyers (names changed to protect the innocent):

“Jen is in full-on reply brief mode.”

“Jen is Bluebooking six ways ’til Sunday.”

“Ben is working.”

“Ben is wishing everyone a healthy, happy, and productive new year. May you all bill enough hours to pay for the partner’s new Porsche. Amen.”

And, from one of Mary Jo White’s minions at Debevoise, dispatched to review documents in Germany:

“William is in Munich this week.”

“William is sleepless in Munich.”

“William is seriously in Munich.”

Poor kid! At least they’re putting him up in a nice hotel.

Facebook lends itself to venting by embittered associates because it is, at least for now, a largely partner-free space. Big Law partners, like a few other types of lawyers (e.g., government lawyers, especially prosecutors), are underrepresented among the ranks of Facebook users.

A few Big Law partners are on the site, but they seem uncomfortable—like when they make obligatory, early-evening cameos at paralegal going-away parties. The reaction of this partner, contained in a status update, is typical: “Philip is still unsure what he is supposed to think about this whole Facebook thing.”

Another, more obvious value of Facebook—and the reason cited by Allen & Overy for lifting its ban—is that it’s an amazing networking tool. Lawyers are relentless networkers. This is because law, even at the highest levels, is not neurosurgery or nuclear physics. It’s a profession where what you know is probably less important than who you know—and who your friends know, and who your friends’ friends know.

Older lawyers join the City Bar and ABA committees left and right, attend an endless series of black-tie benefits at Cipriani, and subject themselves to that unpleasantness called golf. Young lawyers can’t be bothered with all that—they’re too busy billing hours. So instead they network from the (dis)comfort of their Big Law offices.

To be sure, other social networking sites, like LinkedIn or LawLink, are more transparently geared toward business development. A Facebook “poke” is probably not going to land you a Fortune 500 client (or even a date for Saturday night). But due to its ginormous membership, the networking potential of Facebook can’t be denied. When it comes to networking, size matters. (It helps that Facebook is just this side of professional respectability. As Jon Fine of BusinessWeek puts it, “Facebook now combines aspects of Saturday night’s MySpace with Monday morning’s LinkedIn.” In other words, Facebook is more fun than LinkedIn, but more sober and dignified than MySpace, whose 200 million users appear to be 14-year-old girls and the 40-year-old men who love them.)

Although I use Facebook primarily for fun, I’ve had several “Facebook friends” who have turned into real-life, professional contacts. This phenomenon will surely grow over time. Right now the networking potential of Facebook is in its infancy, since its users tend to be relatively youthful. But if it can hold on to them as they evolve from young associates into law-firm partners and general counsels, then it could become a major networking force within the legal community.

Another reason lawyers adore Facebook is that it allows them to reclaim their individuality. The typical lawyer at a Gotham megafirm goes from being an interesting individual—with opinions, idiosyncrasies, likes and dislikes—into a cog, albeit a well-paid one, in a giant machine. So what to do, when you’re feeling like an utterly fungible device for the production of billable hours? Join Facebook!

At your law firm, you may just be timekeeper No. 2141. But on Facebook, you’re an Individual, with Activities and Interests and Favorite Quotes. You can spout off about the Petraeus report, post photos of your rock climbing trip, or review 3:10 to Yuma, for all the world to enjoy. Occasionally someone will comment on something you’ve posted, conveying the illusion that someone actually gives a damn.

Perhaps the best explanation for Facebook’s popularity, though, is the simplest: it’s a great procrastination tool. Anything that allows lawyers to entertain themselves while sitting in front of computer screens and looking diligent will be a runaway hit.

And now, if you’ll excuse me, I have to go update my Facebook status: David is trying to think of a good ending for this piece.