I'm a paying user of yours so I don't have any specific problem with you or historious, but I don't like that you're offering a free month here. I'm sure it won't really have any impact, but in theory by rewarding people who say "have you tried <product>" in a HN comment you're incentivising people to make those comments, essentially offering people the chance to use a referrals scheme without it going through referral links.

That's true generally, although it doesn't really apply in this case. I don't usually give free months to people who mention historious, I was just happy to see a mention and gave a freebie.

Arguably, the free bookmarks you get through referrals are more useful, since they don't expire...