From the National Council of Nonprofits:
More than 37,000 concerned individuals and organizations submitted comments on the proposed gift substantiation regulation, and virtually all that are viewable expressed a common theme: it is a very bad idea for nonprofits to be asking for donors’ Social Security numbers, maintaining that personal information in their files, and submitting it to the IRS. In the view of many, “never is the better answer” when the question is whether individuals should give their Social Security numbers to people claiming to be soliciting on behalf of a charity.
Read more here! (second article)