Hi, I have ads running on three subreddits right now, the smallest one has only 1,460 readers. Considering the CTR (click-through rate) I guess putting ads on subreddits smaller than that might not be worth it, even if that ad has been live for over a week it's still at less than 400 impressions/views.

As a reference, I have an ad on the /r/programming/ as well with 678,120 readers, that one has 113,613 impressions/views during the same time period.