Ask HN: How to gain users?

I've always felt like there was some sort of mystery around companies gaining users. People tend to say stuff like: "it went viral", "everyone just wanted to use it" etc. etc.

It just seems odd to me that some companies succeed in growing a userbase where others just die when most people never even heard of them.

What do you do when you launch a product? Share it on Facebook and Twitter? Do you buy google ads or do you just sit around and wait for people to visit, and if not, do you just move on?

To those who've had a successfully growing userbase at their company: What did you do in the early beginnings to attract users, what was your expectation before these actions and in hindsight, what did real trick?