Adding repost tag is a good way to communicate it. The thing is that most of curators don't read the posts and if they do read some part of them, they seldom look at the tags at the bottom.

The repost thing, even if mentioned is currently considered farming. There was a crosspost feature added few months ago by one community but recently they decided to remove rewards acquired on crossposted posts because of spam and exploitation.

Since we moved to Hive, there have been at least 50 users found who started farming with effortless posts by recycling their old posts. Once certain user does it, others follow as they think that it is acceptable.

In the end everyone would start doing it when it becomes a norm and the platform would become one big pile of recycled spam with no value added. That would go completely against Proof of Brain on which the platform was designed upon.

Facebook cannot be compared to Hive/Steemit. This is a completely different platform. Just as Twitter is.
Also no one is rewarding anyone on FB, Twitter, Instagram like it is on Steemit/Hive.

Hive is made up of at least 70-80% of Steemit user base. Basically vast majority of users moved to Hive. Your old posts from Steemit are part of Hive blockchain. Just exchange steemit.com with hive.io in url. Steemit has become part of Hive history due to shared blockchain up until Hive emerged. All have been reported by the community being angry that someone just spams with effortless posts they already created and got rewards for from the community that is mostly part of Hive community now.
There is no issue in posting the same new post both on Steemit and Hive at the same time, of course. Recycling only relates to posts published on Steemit before it forked to Hive.

I could do it myself. Start recycling hundreds of posts that I have created within years and just farm rewards. I have never recycled a single post.

Artists have been certainly ignored and undervalued. The work is hard and their art has been under-rewarded but I do not think that art related posts should have special licence to recycle.
I don't think it is really a big deal if you repost the same art but add a paragraph or two of new, original thoughts about the content being reposted. This would mean that you add something new to the content.

Hive is miles away from Steemit. There are no more bidbots and many vultures have been eradicated. It is driven by the community in much more efficient and creative way.

I hope that my explanation helps you to understand that problem.