"The ongoing survey asked the same 8,000 families about their charitable gifts made in 2000, 2002 and 2004. While the total percentage of households that gave was similar in all three years (67 to 69 percent), it was not always the same households. The study found that a fairly large proportion of all U.S. households -- nearly one third -- shift between donating and not donating."
Contribute - The People And Ideas Of Giving:
"Levis’ survey shows that most nonprofits post an average gain of just 10 percent each year: they lose 52 percent of their donations, which is then offset by a 62 percent gain in new or upgraded donations. In short, says Levis,nonprofits are losing almost as much as they’re gaining, pouring a river of money into a nearly open drain."