Directors give no time to no-name charities. (Governance).

The first time Steve Greenberg, president and director of Net2Phone, a $150 million Internet phone service provider in Newark, N.J., visited the Museum of the Jewish Diaspora in Israel 20 years ago, he did what all tourists do: He strolled through the museum's dioramas of Jewish life before the Holocaust, researched his ancestry in a genealogical database and lingered, as he would do on 50 subsequent trips, among miniaturized models of the world's great synagogues. When Greenberg was invited to join the museum's board five years ago, he accepted immediately.

The museum is probably lucky to have him. With an annual budget of about $3 million, it's small and doesn't rank among the world's best-known institutions, while similar nonprofits are suffering a scarcity of board help. Many American CEOs wouldn't commit to the long trip Greenberg makes to Tel Aviv twice a year, even in less volatile times. Some CEOs can't get away from the office. Others are cuffing back on board service at the request of their own directors. All in all, the same dearth of directors that plagues the boards of public companies has hit small nonprofits even harder.

A decade of explosive growth of new U.S. charities, combined with a stick-to-your-knitting economy, has exacerbated the shortage. Worst off are organizations with budgets under $10 million, which account for 95 percent of all U.S. nonprofits. As many as 3 million board seats are collecting dust, according to a report by the consulting firm Booz Allen Hamilton and Volunteer Consulting Group, a New York nonprofit that matches executives with nonprofit boards.

Part of the problem is that smaller nonprofits just don't have the sex appeal of the brand-name institutions, says Barry Jaruzelski, vice president of Booz Allen. "People want the stature that comes with being affiliated with the Museum of Modern Art," for example, he says.

But that attitude doesn't hold water with Greenberg. "When you become CEO the demands on your time become huge, but if you use the excuse that you don't have time anymore, you don't have your priorities straight," Greenberg says. "I don't think of it in terms of what I'm going to get back for myself or for Net2Phone. I think [the Diaspora museum] is an important place and I have an ability to help it."

The director shortage is an opportunity for up-and-coming minorities, women and junior executives. Interested candidates can register at boardnetUSA.org, a new Web site run by Volunteer Consulting Group that pairs candidates with compatible nonprofits.