By targeting recent visitors to a subreddit... they are essentially no longer targeting anyone.

I rarely browse the reddit homepage, but I know that most redditors do.

And what's on the homepage that most everybody who looks at it clicks through to? Any and every random subreddit you could think of.

I have seen /r/bitcoin on the homepage recently.

I have seen /r/security on the homepage recently.

I have seen /r/rtlsdr on the homepage recently.

That's ridiculous.

If I want to advertise a SDR to the "rtlsdr" crowd, I do not want to pay for impressions for every single user on reddit who has just recently viewed /r/rtlsdr because they had a quirky post which resonated with people.