Marketing, marketing, marketing. There are lots of chat apps and they all have users even if you never heard of it.
It was easier to break in with a new chat/message app, for example, years ago as long as it had innovative technology, like WhatsApp being cross platform or whatever it offers.
Twitter exploded because it had great word of mouth at a big conference and users saw that you could keep instant tabs on the statuses of other users and it spread “news” more than the other sites out there at the time. Once Twitter showed its power with users breaking major news before TV/newspaper sites and celebrities set up accounts, there was no turning back. Reporters started to write articles when athletes tweeted something. Anyway, you’d need connections to get into such a conference.
If you break in now with a new platform, you need a gimmick based on filling or anticipating the needs of the users. For example, Signal’s gimmick is that Edward Snowden gave it its seal of approval or FrozenChat, which has off the record chats for privacy. You could also market yourself as being “troll free” (ha) or have a different TOS than Twitter or Facebook or whatever, to try and pick off disgruntled users.
In addition to the technology that works, it would be best to get feedback in the tech/programming communities, give the geeks something they like and helped test for you.
Word of mouth today comes from social networks like Reddit, Youtube, etc and the programming forums, tech websites/podcasts, and you’d need people to preview and review your app to hype it up.
You don’t need all of the market, just a nice niche. Maybe one day you’d be bought over.
The teens I know love Snapchat and WhatsApp, and maybe new apps I have not heard of. All it takes is one of their cool friends to use it. People like shiny new toys which fill a gap. These teens are the end user demographic, who were not involved with the initial testing.
Regarding Google’s Search Engine Optimization (SEO): most apps don’t need that. Most people don’t type “microblog” or “social app” in Google to find one. You’d need to market it at the aforementioned tech forums/blogs/videos/podcasts so people download your app from the Apple or Android stores. You could rig the SEO so the articles, videos, and forums talk about it, but SEO is not as important as it used to be regarding apps. Like why would a prospective user type “Samsung Galaxy Apps” in Google instead of his/her mobile’s app store? Most people get apps from their store, so that is were the focus must be with the marketing. Google will index and rank your site based on traffic, backlinks, content, format, and other algorithms.