They send out thousands of 'micro' payments to randomly selected addresses. These payments are all sent from one key and this key has been tagged with their website address on Blockchain.info. So they hope that these thousands of users will look up the payment, which most will do as they don't know where it came from, and when doing so see their website address and go to their site and possibly sign up for the services/products they have on offer.

So essentially the same as a bulk spam email going out only here there is some cost involved.