It wasn’t until his mother, Irene, passed away in December 1986 that Stuart Waxenberg found out about Josef. In emptying the contents of his mother’s safe deposit box, he found a card listing the gravesite of Josef Kleizman, a maternal uncle he had never heard of before.

“Mother never talked about it,” he said.

Waxenberg later learned that Josef had died in 1921 at age 6. Like hundreds of other recent immigrants, the family was too poor to pay for Josef’s burial, so the Hebrew Free Burial Association buried him in Mount Richmond Cemetery in Staten Island.

This past October, Waxenberg visited his uncle’s grave and did what he wanted to do ever since he learned of his mother’s dead brother — he brought dirt from his mother’s and grandmother’s graves, located in the Hebrew Cemetery in Rock Island, Ill., and placed it under the headstone.

Then he purchased a headstone for $150, “to help someone else,” he says, as a sign of gratitude for the anonymous donor who had paid for Josef’s headstone and burial decades ago.

Most recently, Waxenberg has donated $180 to the HFBA — an organization located 1,000 miles away, which he had never heard of until his mother died — to set up a permanent Kaddish for Josef. “I always send in money during holiday times and on his yartzheit,” he says.

Waxenberg represents a group of “loyal donors” whom nonprofits should be courting, but generally are not, according to Adrian Sargeant, Robert F. Hartsook Professor of Fundraising at Indiana University’s Center on Philanthropy.

“Nonprofits should be targeting people who have benefited from their services,” says Sargeant, whose latest book on the subject, “Fundraising: Principles and Practice,” (Jossey Bass), will be published next month. “Sometimes nonprofits are a bit shy of doing that, but a lot of people, when they’re in the position to give back, are happy to do that. There’s no reason to feel uncomfortable.”

Donor loyalty is an issue the nonprofit world is failing at, Sargeant says. “For every six donors we recruit, we lose five,” he says. And well over half of the people who give an initial gift don’t give a second. “If you were running a business and lost half of your customers, you wouldn’t survive,” he says.

What’s more, Sargeant’s research indicates that a 10 percent improvement in loyalty can yield up to a 200 percent increase in the projected value of a donor base, as significantly more donors upgrade their giving, give in multiple ways, recommend others and, ultimately perhaps, pledge a legacy/bequest.

“Perhaps the message is that even within the economic downturn, people still attach significance to the yahrtzeit and give a donation — even if it’s in a smaller dollar amount — in memory of their loved ones,” says Andrew Parver, director of education and outreach at the HFBA.

People who engage with an organization in multiple ways are more loyal than those who don’t, Sargeant’s research indicates. So it’s probably not surprising that service beneficiaries who then become donors tend to remain loyal supporters, even when the tough economic climate necessitates cutting back.

Just ask Gregory Trakhtenberg, a Queens resident who emigrated from Odessa when he was 3 years old. Interest-free loans totaling nearly $7,500 from the Hebrew Free Loan Society helped him afford physical therapy school. Ever since graduating in 1999 and opening his own physical therapy practice, he has been giving back to the organization.

“I give a few hundred dollars a couple of times a year, every year,” he says. “There are millions of charities out there, but I personally experienced what [Hebrew Free Loan Society] did for me,” he says.

“With them, I’m sure where the money is going and that definitely makes it easier for me to donate to them, since I’d like other people to benefit from them,” he says.

In recent years, the HFLS has experienced success in its efforts to solicit donations from those who have paid off their loans. In 2007, 5.5 percent of the 847 loan recipients gave a gift. The percentage has been increasing each year, with 7.4 percent of borrowers becoming donors in 2009.

“These numbers are conservative, because they do not reflect repeat givers and those who started giving before we kept that data,” says Moshe Soloway, the assistant executive director of the Hebrew Free Loan Society. “So in reality the universe of borrowers who became donors is bigger than what you are seeing here.”

While a majority of these gifts are small, they are consistent. “It’s not Sergey Brin,” admits Shana Novick, HFLS’s executive director, referring to the Google co-founder who gave $1 million to the Hebrew Immigrant Aid Society in October 2009 in recognition for HIAS’ assistance to Brin’s family when it fled the former Soviet Union three decades ago.

Still, HFLS provides loans to many Russian immigrants, where “there was no culture of philanthropy or volunteerism,” she says. Encouraging beneficiaries of the interest-free loans to get involved in philanthropy has been a goal of the organization.

At the HFBA, the closest “Sergey Brin”-like giver was actor Mel Brooks, whose maternal grandparents, who went by the name “Brookman,” were buried by the HFBA. When HFBA officials contacted Brooks about it in 2008, he paid to fix his grandparents’ tombstones, sponsored four tombstones for unmarked graves, gave his grandparents perpetual care and Kaddish and even rounded up his donation to the next $100, insisting that the remainder go toward an office lunch. “It was really sweet of him,” says Amy Koplow, executive director of the HFBA.

Still, most loyal donors are less like Mel Brooks and more like Iris Sadove, who was orphaned at age 16. As she was washing her hands upon exiting the cemetery, her uncle dropped a bombshell. “By the way, you were adopted,” she remembers him telling her. “It wasn’t the best timing,” she admits.

Years later, as a single mother raising three children, Sadove hired a private investigator to help discover the identity of her birth mother. Soon she received the bittersweet news: her mother was identified as Brigitta Blumstein (she had remarried), but she was no longer alive. She had been buried by the Hebrew Free Burial Association.

In July, Sadove visited her mother’s grave along with her half brother, Charlie, whom she discovered while verifying her Jewish lineage in order to make aliyah. It was a rainy day, but the sky opened up and the sun came out just as the rabbi finished reciting the memorial prayer, El Maley Rachamim, as the reunited half siblings prayed at the foot of their mother’s grave.

Sadove was overwhelmed with gratitude and wanted to pay for her mother’s gravestone, but was told that someone else had already done so. “It was so touching to think that she would not have had a proper Jewish burial, and probably none at all, if not for the work of the Hebrew Free Burial,” she says.

Instead, she donated headstones for other unmarked graves. Whenever she gets a mailing from the HFBA, she gives a donation, she says. “If not for Hebrew Free Burial, my mom would be a pile of ashes. I feel very blessed to have so many piece of my broken life coming together.”