Back in 2010, when Pinterest just started, it had only about 3000 registered users. Such amount was gained in first 3 months after the launch, which is sad to say the least.
Instead of changing the product or the way it works, CEO of Pinterest started finding passionate people like himself. He ran meet-ups and small conferences in boutiques, cafes, etc. He invited people to take photos and then share them, also working with bloggers to spread the word about Pinterest and drive traffic to their blogs via image posts.
Long story short, the idea of this growth hack is the following – try to engage with the active users, and let them spread the word. It is always easier to work with the existing active users, rather than acquiring new ones and making them “love” your product.