We used this tactic as a growth hack in our early days (2016), and it worked great!

It also inspires sharing on Twitter, which helps drive the recommendation flywheel:

https://twitter.com/tdnvl/status/722882182759452675

The trouble is that it isn't easily scalable in-house at high volume. It was great until we realized that we were managing a large scale mailroom. But by then the flywheel was spinning ...