There are lots of ways data is shared.<p>For the vast majority, it&#x27;s demographics and interest based stuff. LinkedIn for example will keep lots of 1st party data to itself for it&#x27;s own ad business but will share generic data like: female, 30s, IT engineer, new york, etc. This is how much of the ad targeting works. Trying to target a single person&#x2F;identity just isn&#x27;t easy, scalable or worth it so big overlapping buckets are used.<p>On the other side, specific identity data is also shared, called PII (personally identifying information), in a hashed format with other data networks. This is often used in retargeting by profile, an example being if a company wants to target all of it&#x27;s current customers, it&#x27;ll upload it&#x27;s CRM database full of emails and data providers will match this up to cookies or other identifiers and let that company target these users with ads online. It&#x27;s anonymized in that the advertising company doesnt know your identity, just that you&#x27;re in this bucket of &quot;XYZ email address list&quot;.<p>The way providers get to know your identity is major sites that share your profile data when you login, because they definitely know that it&#x27;s you. LinkedIn will set a cookie when you login and then they&#x27;ll have an API or data dump to other providers that can request your info or if you fit a bucket (in a hashed format).<p>Data is usually on a CPM (cost per 1000 impressions) basis although ranges widely from $1-$100 depending on quality and depth of targeting.