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There was an invite system earlier put in place by https://steem.ninja, which went dead, presumably, after the hive breakup.

That explains it; because I quite remember that I was referred by someone. Thanks for the clarification anyways.

If you are talking about a referral system like the one they have on https://hiveonboard.com, then there is none that I am aware of on steem blockchain.

Okay, thank you very much.