More and more people are using online social networks, such as Facebook and Myspace, and it’s not just kids logging on. According to a recent survey from the Pew Internet & American Life Project. That has big implications for charities, says the fund-raising consultant Roger Craver.

Use of social networks by people 18 and older has gone up four-fold since 2005, from 8 percent of adults, to 35 percent in December of last year. (The numbers still skew young however. While 75 percent of adults age 18 to 24 have profiles on a social networking site, only 7 percent of people 65 or older do.)

Mr. Craver suggests those figures mean that there is a “new math for fund raisers.” On his blog The Agitator he suggests that charities with at least 100,000 donors should be able tap social networks to raise more than $500,000 for an “urgent project or need.”

First, based on his own survey work, Mr. Carver figures that 15 percent of donors can be considered “missionaries” for the cause—people who actively recruit new donors. Out of 100,000 donors, that makes for 15,000 “missionaries.”

And since Pew found that about a third of adults are active social network users, that means 5,000 of the missionaries are on Facebook or other such networks, he estimates.

If you ask those donors to help raise money for a special need, Mr. Carver figures that half will agree to do so. These 2,500 donors, he goes on to speculate, will then reach out over their networks to 10 friends each. If half of those so contacted give $35, that raises $437,500. If you figure the missionaries themselves give $35, you have another $87,500, for a total of $525,000.

“Return per personal fund raiser = $210.” Mr. Carver writes. “Total cost = negligible.”

Mr. Carver invites readers to “pick this scenario apart,” and indeed some did, prompting him to write a second blog entry defending his numbers and inviting readers to test his calculations.

What do you think of Mr. Carver’s math? What have been your experiences fund-raising through social networks?