Evelyn Rodriguez' notes on Google product management barely mention it, but Google's marketing primarily consists of generating "network effects" (word of mouth). I don't think I've ever seen any conventional advertising for Google or for any of its products.
An example of a successful marketing program was for Google's web-based e-mail, called "Gmail". During the lengthy beta phase, and even to some extent now, you had to receive an invitation to sign up for Gmail. Google granted users a certain number of invitations that they could pass on to their friends and co-workers.
You might think that the best way to market a product is to make it as available as possible. Some reverse psychology comes into play, however. It quickly became somewhat of a status symbol to have a Gmail account, to the point that people were selling Gmail invitations on eBay.
Companies bombard prospective users with available products every day; why should a user consider trying a new product? If you hear about a product from someone you trust, you're much more likely to try it. You may also be intrigued if the product is somewhat "unavailable" to you, wondering if it must be so good that the company doesn't even need to make it readily available.