Marketo has done company-level tracking for years[0], and if you click through f...

Marketo has done company-level tracking for years[0], and if you click through from an email or fill out a form they can keep tracking you as well as back-fill any previously anonymous visits you made (depending on your browser cookie settings, of course). Once it's in the system, they partner with a number of companies, some of whom can help populate contact data[1], eg: "over 1.5 billion opt-in email addresses" -- how plausible is that? They have as customers a few companies[2] you're likely familiar with (eg: VMware).