I voted for Reddit for a few reasons:

the ads there seem targeted quite well generally, you seem to be able to select which communities/subreddits you want to target with the ads and generally reddit is quite similar to hive in terms of community niches.

We'd still need an easy way for people to create accounts and try being active here, potentially even front-ends to ask new accounts a couple questions and suggest some authors/communities to subscribe to at account creation, but generally redditor's could have an easier time getting accustomed to things here.

Close second would be youtube, but there we'd need to be clever with the ad and it may cost more if we opt for a video ad which would require even more uniqueness and grasp on viewers.

Facebook I'm not too familiar with for a long time now.