Doximity pretty much has a monopoly on doctors' eyeballs in the U.S. -- at least outside the electronic healthcare records space -- and the question for investors to ponder is just how valuable that market is. The company estimates its market opportunity at $18.5 billion. That's composed of $7.3 billion in pharmaceutical marketing to doctors, another $6.9 billion in job placement in healthcare, and a $4.3 billion opportunity in software telehealth.