Of those who did make additional gifts after an initial online donation in 2006, according to 12 organizations offering data in January, 37 percent never gave another gift via the Internet, while 18 percent gave electronically in one year and through other channels in another.

Charities have been using the Internet for fund-raising for more than a decade with mixed results.

“People are asking us all the time why we don’t reduce mailing costs and save paper with online fund-raising, but the simple fact is that people come online to give a gift once and don’t repeat,” said Jennifer Tierney, development director at Doctors Without Borders in New York, which took part in the study.

The 24 nonprofit organizations had 9.5 million donors and total revenues of $747 million.

Charity fund-raisers offered several possible explanations for the study’s findings. Many donors using the Internet to make their first gift to an organization are responding to a disaster like the Asian tsunami or Hurricane Katrina, and those givers may not be interested in supporting a group’s continuing work.

Another issue is that nonprofit groups tend to add donors acquired online to their direct mail lists, which encourages those donors to give in more conventional ways.

“Direct mail may not be a Maserati, but it’s very effective because it is very highly evolved,” said Lori Held, membership marketing director at Trout Unlimited. “We know how to ask for money using the mail, but most organizations are still trying to figure out how to do that online.”