Couple of interesting answers on Quora, but this part from Michael E. Driscoll’s answer is quite interesting:
You could even get 100s of daily TBs of data yourself: if you can afford
the network bandwidth fees, there are ~100 marketplaces (Twitter’s MoPub,
Google’s AdX, Facebook’s FBX to name a few) that surface approximately 200
Billion advertising auctions per day. You can build a bidder, get a seat on
their exchanges, and make millions of daily trades — you’ll just need to
convince a brand to act as their broker, and take your 20% cut of spend.