Posting your product indiscriminately, as is the premise of the OP, is spam.

Promoting your product via Twitter or your friends is fine. (promoting a submission on Hacker News [i.e. highlighting the submission itself in an attempt to vote manipulate] is against the site rules, which is not spam but still bad)

But it's not necessarily indiscriminate. Many of these are sites that want submissions. Even Hacker News is OK if you're doing something relevant to the audience.

As an example:

https://news.ycombinator.com/item?id=14202456

Edited my comment to clarify that voting manipulation is bad, not Show HN itself. Show HN of course is fine.