The choice between Mobile and Web development usually depends on several factors: your target audience, the key features of your app and your personal development skills. If you're aiming at providing a rich native experience, having access to mobile-specific features, or targeting users who primarily use mobile devices, mobile development might be the way to go. This, however, requires you to learn specific languages (Swift for iOS, Kotlin for Android) or cross-platform solutions (React Native, Flutter).
On the other hand, if you want to reach a wider audience regardless of device, provide a more general experience that doesn't rely on mobile-specific features, or if you're more comfortable with web technologies like HTML, CSS, and JavaScript, then Web development would be a better fit. Additionally, web apps are typically easier to maintain since you don't need to develop and update separate apps for each platform. A progressive web app (PWA) could give you the best of both worlds to some extent – the broad accessibility of the web and some of the device-specific optimizations of native apps.
In terms of social media apps, I'd say both Mobile and Web have their own place. Many popular social media platforms provide both web and mobile app interfaces (e.g., Facebook, Twitter, Instagram), as both forms offer different conveniences to different users. As a startup, you could start with one (the one most suitable to your target audience and your skillset) and expand to the other once you've established a user base.