Reddit was the only thing that resembles social media I ever used. Was a long time RiF user, as I absolutely hated the default interface. Even moderated a couple of subreddits back in the day (although I sort of dropped Reddit in the past couple of years, so I may be out of the loop).

My fellow mods and all prominent users I interacted with (the vast majority of them not from tech as it was not a tech focused community) were all well aware of 3rd party clients, and many used them.

This is very anecdotal, but amongst Reddit more "intense" user base, I would be surprised if 3rd party client usage was low.