 It quickly became the means by which most Americans learned about and first provided financial support for their charities of choice.The explosive growth of the nonprofit sector in the United States — quadrupling in the 1980s and doubling again in the 1990s and early 2000s — led to a massive expansion in the use of direct mail to build and sustain large, nationwide donor and membership lists.Today, direct mail fundraising accounts for at least one-fifth of the more than $250 billion contributed annually in the U.S. to the nation's 1.6 million nonprofit organizations.