If what Im gathering from the website is correct the app is for sharing actual stories - aka long narratives and written stories about events that have happened in people’s lives. I know one of the other comments said they don’t see the value since they can share their story on any social media site, so I think there’s a disconnect in the message between “story” as in a written story or “story” as in IG/TiKTok 24hour stories. The only thing on the website that got me to understand you mean literal stories is the pictures which all feature long post.

Anyways that being said I think you’re actually much more niche than 18-30yo, and you should be targeting people who are interested in story telling, creative writing, blogging, maybe even theater… basically people who value the art of sharing and reading long content post. With this you need to figure out how you can make your website & marketing strategies more efficiently target and appeal to these people. From there you can find places these people hang out, maybe book clubs for younger people, flyers in coffee shops and libraries, etc. Maybe also micro influencers in your city, people who have their own blogs - fashion, food, travel, parenting - since if they get on you can pitch it as another way for them to tell their story/grow their brand, and if they get their viewers on they will also enjoy sharing/reading stories.

Unfortunately targeting a university as a whole won’t do you any good. College students who love tiktok/ig/twitter aren’t your market - they’d rather tell their “story” verbally on their 24hr story as a “rant” or in short hand writing as a post, and not take the time to write out and read their friends. However, if you want to target a specific group at university I’d recommend finding book clubs, creative writing clubs, maybe school newspaper team, and pitch to them to get on the app and use it. Again value proposition that it’s for people like them with their interest.

Long story long you just really need to hone in to your target audience and define them as specifically as you can, and then it’ll be more clear how you get them to join. Yes your goal can be to eventually be to be used by all young people across the globe, but you need to start smaller and be super specific so you can get a strong foundation which you can continue to build on. I think your ideas of having contest are great and would engage your target audience, but again you really need to understand who your target audience really is, who would use the app, and who will take the time to share these stories. Because as someone 18-30 it’s definitely not everyone who’s 18-30.

But your users are there, you’ve had organic growth, there is a market and i’m sure you’ll find it! i believe in you! keep hustling and get it done!

(i know it’s such a long response i’m in a similar boat building and marketing a niche social media app so ive been learning a lot in the process)