Ad companies like Google run on your digital exhaust. They make it easy to embed their analytics, login, payment, crash reporting, and other tools into websites and applications.

This data is collected and combined with their other data sources. They know your physical location via GPS or approximate based on your internet provider. If you use Google DNS they know all the websites you are going to and when. Not using Google DNS?, That's cool, most websites have some component that integrates with Google. Use android apps? Almost all of those use play services, so Google gets that info too.

Hanging out with your friends tonight? Ad companies can use location data to figure out who you hang with and build a social graph of people who are around you all the time. It's almost Valentine's Day.. better buy a gift for that person you live with. Here are some suggestions based on what they have been looking at.

They own most people's email accounts, they know what you watch on YouTube, they know about your preferences, they know what you might buy and when. They are a search company so they will ingest any and all data about you on the web. If you bought it with Google pay or see an email receipt they know if their advertising is effective.

On top of that, now this data is all fed into machine learning algorithms to further mine the dataset.

They know more about you than you do.