Slither.io uses ads on the web and mobile versions, with in-app purchases on the mobile versions to play without ads.
The games generally have short play loops where ads are shown every time you die, so the number of impressions per session can be fairly high. The game cycle is pretty addictive and simple to pick up, but has a little depth to it.
Slither.io is incredibly successful for a web game ($100k/day[1] has been frequently quoted in the press, but I don't know the source of that and haven't seen more recent stats).
There is definitely money in web games, but it's often down to luck like any game promotion. Slither.io got huge traffic from vloggers like PewDiePie playing the game multiple times[2][3]. You also need quite high unique visitor counts before you can use certain ad networks. (The last mobile web game ad service I looked at was asking for around a million impressions a month before you could sign up.)
