Sandberg at the 2011 MPW Summit Credit: Asa Mathat

We asked the question a few months ago: How can it be that Facebook, whose biggest and best base of customers is female, does not have a single woman on its seven-member board of directors?

Yesterday, it happened. Facebook COO Sheryl Sandberg got named to the board. A smart move, and long overdue.

It’s smart too because investors are looking for signs to be confident in Facebook , whose stock now trades around $32 vs. $38 when it went public in June. Sandberg happens to be the Facebook executive who, more than anyone else there, has professionalized Mark Zuckerberg’s eight-year-old social network, figured out how to make money, and lured heavy-duty advertisers.

Yet even as many are cheering the decision to add a woman to the board, we also have to wonder: Can Sheryl Sandberg do it all?

Besides being Zuckerberg’s No. 2 and most critical recruit–from Google in 2008–Sandberg, 42, is also a board member at Walt Disney , a director of several non-profits, the wife of a CEO (Dave Goldberg of Survey Monkey), the mother of two small children, and the world’s most vocal evangelist for aspiring career women.

Whew! By the way, Sandberg quit the Starbucks board in order to facilitate Facebook’s IPO amidst all her other duties.

Now you may ask whether I’d be questioning Sandberg’s capacity if she were a man. Here’s the thing: There are supermen who have Sandberg’s bandwidth and can match her in the business world, but they probably have a wife at home who cares for the kids and helps maintain their sanity. As I noted yesterday, Anne-Marie Slaughter, the author of the explosive Atlantic magazine cover story “Why Women Still Can’t Have it All,” contends that Sandberg too harshly judges women who are not as ambitious as she.

Sandberg ranks No. 12 on the Fortune Most Powerful Women list. The higher she climbs, the more she will draw controversy. (That’s the reality with powerful women.) And yet, for all the scrutiny and portrayals of Sandberg as a practitioner and proponent of women “having it all,” the media may not have noticed: Sandberg actually doesn’t talk about “having it all.”

“I never use the term ‘having it all.’ Because ‘having it all’ makes everyone feel like they’re missing something,” Sandberg has told me. While she declined to comment on her board appointment (or on the Atlantic story), she and I have talked often about women and power, and she has said: “I don’t believe in ‘having it all, but I do believe in women and men having both a successful career and family. The more women we get into positions of power, the more likely we’ll get that.”

Her Facebook board seat is yet another perch for Sandberg. Now it’s up to her to use it wisely.