That reminds me, there was this panel I tested in the marketplace a couple months back that I've been meaning to find again. It was a YouTube-only panel, meaning they only had YouTube services.
The service claimed to have real followers like many others, so I used it on one of my YouTube channels to test this claim.
Sure enough, the followers I got from that service 2 months ago are still engaging with the videos on my channel to this day.
Unfortunately, even if I find the panel, I can't endorse specific marketplace services here because of my position.
Edit: I found it, but the guy never paid for the thread to go live. That happens surprisingly often, if you can believe it.