I noticed quite a while ago. I haven't checking in on Steemit like I used to but I do remember the leadership at the .com talking about bringing adds for several months now. There was even a brief survey about it I filled out back then.

I think one of the reasons was to help pay for the servers to host this website since the price of steem got so low. They said it cost millions per month to run this place. They dropped 75% of the staff in the corporate office too. If the price of steem goes way up again maybe they will remove the adds.

I am not sure if the adds are visible from any of the related apps like busy.org or such, due to the fact I only use this website for all my steemy fun.