The 29-year-old newcomer to Starbucks' board

This past summer, when Facebook COO Sheryl Sandberg emailed me about Clara Shih, we at Fortune knew to keep a lookout.

"I think she is awesome," Sandberg wrote in her email.

Sure enough, Starbucks (SBUX) yesterday named 29-year-old Shih, a social-media entrepreneur, to replace Sandberg on its board of directors.

A 29-year-old on the Starbucks board?!

Starbucks is bulking up on social-media expertise at a time when boards of most Fortune 500 companies desperately need that know-how. Starbucks knew about Shih because the company is a customer of Hearsay Social, her two-year-old startup that gives businesses tools to monitor their employees' activity on social sites like Facebook, Twitter, and LinkedIn (LNKD).

Sandberg knew Shih because they worked at Google (GOOG) at the same time, before both shifting their aspirations to Facebook. Shih built Facebook's first business application, published a best seller (The Facebook Era), and launched her social-media company--all before age 29.

Hearsay has attracted more than $21 million in funding from the likes of Sequoia Capital, where Shih dazzled partner Bryan Schreier when she came asking for money last year. "We meet with 1000's of entrepreneurs each year but get into business with only about a dozen," says Schreier. "Clara made such a strong impression that we offered her a term sheet 24 hours later."

Schreier calls Shih, who arrived in the U.S. from Hong Kong with her parents 25 years ago, "a triple threat." He explains: "First, she has a computer science degree from Stanford. Next, she literally defined the industry of social marketing (wrote a best-selling book) and is now the most sought after keynote speaker on the topic. And third, she is an absolutely outstanding CEO of the fastest growing SaaS company I am aware of."

For us at Fortune, it was pretty much a no-brainer to name Shih one of our Most Powerful Women Entrepreneurs this year.

In her application for the MPW Entrepreneurs award (we award 10 entrepreneurs annually and host them at the Fortune Most Powerful Women Summit), Shih told us that she works "100 hours" a week. Hmm.

Then we witnessed what a high-functioning do-it-all entrepreneur she is: Shih married her neurologist boyfriend on October 1, two days before the Summit, and skipped her honeymoon to attend our three-day event.

We seated Shih at Warren Buffett's table opening night at the Summit--the least we could do for this tireless up-and-comer. In her MPW Entrepreneurs application, Shih wrote that her poor immigrant family had "very little besides high hopes and big dreams" when they came to the U.S. in 1986. "My entire life, I have viewed every problem as an opportunity--I've had no choice," Shih told us.

Obviously, Shih is listening to her mentor, Sandberg, who tells young women looking to succeed: "Do not lean back. Lean in."

Read my colleague Colleen Leahey's profile of Shih in the 2011 Fortune Most Powerful Women Entrepreneurs gallery. And click here for an interview that Fortune's Dan Primack did yesterday with Starbucks' newest board member.