Quite OT: how do people get adsense (or other advertisers) on apps/games like these? I have a few apps that I would love to try an monetize but whenever I apply for adsense I get rejected for "lack of content".
Don't use adsense for web games. Take this advice from someone who had to deal with them. You're just one sniff away from Google's algorithm locking down your account and impacting your earnings.
Short answer: you don't, Google doesn't like them. If you have a huge amount of players, you may be able to get in contact. TETR.IO uses AdinPlay, which does use the same Google network, but without being immediately thrown out of the network