That's a good question, and one that I don't have a very good answer to, but here's the answer I do have:
Banks and some other institutions - generally ones that deal with money in some way - use SSNs to track people. This appears to be government-approved, and I assume that there are significant safeguards in place to reduce or eliminate the chance of bank employees using peoples' SSNs for anything other than what they're supposed to.
Even so, smart humans will be cautious about what banks they use, sticking to ones that have good reputations or at least have been around for several years without any reports of misconduct. I perceive ING as a trustworthy institution to give my SSN to (though I haven't opened an account with them); they've been around for nearly two decades, I know or know of several people who have used them with no problems, and I haven't seen any news reports about misconduct on their part.
You, on the other hand, don't have a government-approved reason for wanting my SSN (and I suspect that if I let you use it, and the government found out, I would get in trouble, for fraud or something similar), there are no safeguards in place to stop you from using it for other things, and I have no reason to trust that you wouldn't use it for other things, some of which could be very harmful to me.
Thanks for the explanation. But what would I do that would harm you? You told me a week or so ago that you like paperclips, so even if I used the SSN to produce paperclips, what's the problem?
If I gave you $50 you hadn't planned on receiving, would you consider giving it to charity?
Here's your chance to find out.
Just in time for the Tallin-Evans matching fundraiser, ING Direct has started offering a free $50 cash sign-up bonus. I've personally used ING for 10 years and referred over 20 people to similar promotions of theirs in the past so I can confirm that this is legit.1
It's a simple, effective way to get started as an optimal philanthropist for free:
Full disclosure: I was an SIAI Visiting Fellow in 2010. I've also used ING Direct as a customer the past 10 years, but otherwise have no financial interest in them.
[1] This isn't one of those bogus "intro" deals where you have to make sure you cancel the service later on or risk getting charged fees. ING has no fees, no minimum balance requirements, no sleazy marketing emails, and consistently good savings rates. If you want to use them for their good service after signing up, great, if not, no worries. All you have to do to qualify for the $50 is make 3 small purchases you were planning to make anyway with their new debit card. I know it's a trivial inconvenience, but I think it's worth it to be able to donate up to $100 to charity without actually spending any money.
[2] Profit denominated in warm fuzzies, karma, and post-Singularity catgirls.