Casual RPer on here, at the best of times, since I've mostly moved on. However, I do have a few active RPs via private messages and a thread that gets revive and replied to whenever me and the other user are up for continuing that one.
I'm not sure if that is a good active count, though, since, as I said, I moved more away from RPing on here for a handful of different reasons (one of which being life has kept me from doing as much RPing as I would enjoy)
When I used to be really active on here, I would do thread or private messages depending on what my partner(s) preferred, since I didn't really care one way or the other. Most people didn't interact with what I posted on the forums, so if they enjoyed the read, then that's great! No reason to be self-conscious about that. (Plus, it let other members get a feel of my RP style and how it would change depending on the partner I was replying to, so helped pair with potential partners whenever I was looking for new partners)