Google will soon let advertisers tap into one of the most lucrative types of ad targeting: email addresses. The search giant is rolling out a new tool called Customer Match, which lets advertisers use a list of email addresses to target specific users across Google services. To be targeted through this technique, you only need to be logged into your Google account and have given your email address to a retailer, perhaps by buying something from their website or giving it out to sign up for a loyalty program. Google says the individual email addresses are anonymized through Customer Match. That means companies won't be able to create personalized profiles of individuals, but they will still be compiling lists with general customer habits like YouTube viewing and Google search histories to target ads with.

For instance, Google will let a retailer you've shopped at upload your email address to Customer Match so that it can serve up ads while you're watching YouTube, searching Google for related products, or checking your Gmail inbox while signed into that email account. The ad product also lets companies create a general audience profile based on existing customers' habits and interests to target new customers when they use Google services.

Customer Match raises some valid privacy concerns

Google rose to the upper echelon of tech titans by amassing a lion's share of the web advertising market. Now, the company is looking for new forms of ad revenue as more consumers gravitate toward mobile devices, where an ad's money-making potential is reduced. Further complicating Google's position are strategic moves from Apple and Facebook to reduce the strength of the web. Apple is doing this by allowing new ad-blocking software on iOS 9, accessible only from using its own mobile Safari web browser as opposed to Google's Chrome. Facebook, on the other hand, has created a robust web and mobile ad service from within its social network while expanding the variety of the web's information, from news articles to videos, users can access right from their News Feed.

By tapping into email addresses, as both Facebook and Twitter do with their own respective products, Google can take higher cuts of the more costly and better targeted ads being served even from within the Gmail and YouTube apps on mobile devices. Google calls Customer Match a "privacy-safe" product, but that may not quell the concern of users who feel that advertisers are getting even more specific with their ad targeting.