One of the main ways they make money is by selling your personal and financial information.
Read a good one on Pogo was Right (WE HAVE MET THE ENEMY AND HE IS US):
Terry Wyatt called his mortgage broker one morning about refinancing - and within hours began getting calls from other brokers and lenders he's never heard of.
... So how did brokers and lenders as far away as New York and Florida know - and know so fast - that Wyatt wants to refinance? Thank the credit bureaus.
When a lender or broker checks someone's credit report, it signals that person is in the market for a mortgage or to refinance. The credit bureaus turn around and sell that contact information to others in the mortgage business looking for leads.
Source - L. A. Times
PogowasRight is a good read for anyone interested in keeping up with privacy issues.
One way to stop the number of times your information is sold is to "opt out."
Information on how to opt out from unsolicited credit offers from places you already do business with, here.
Information on how to opt-out from places you don't do business with, here.
If you do not opt-out with places you already do business with and respond to their privacy notices (interesting way to classify them), they can and will sell your information. Of note, some of these notices are hard to respond to, or even understand what they actually say.
Here is a post, I wrote, which explains this further:
Warning if you don't open (and respond) to snail mail from American Express, they will sell your personal information!
You can opt out from telemarketing calls (one of the end results of your information being sold), here.