What a coincidence. I have just launched a social media web app where all the posts vanishes at midnight. I am facing a similar challenge in growing my users. I have decided to advertise and pay a group of University students to become Ambassadors and spread it with their University.
That is how the likes of Facebook and Snapchat started.
In fact, research shows that it is better to have a group of 50 people who are connected in say a University to use your app intensively and spread it among their peers, than have 1,000 unconnected users.
https://preview.redd.it/k1m1j583lg3h1.jpeg?width=1080&format=pjpg&auto=webp&s=f7951e6c727d5175b806f51854381c84cf6e4cf7