There are pros and cons of each approach, for example a native mobile app would be better suited for apps that require users to shoot and post photos and video quickly and easily, but a mobile web app might be better for an 'app' where you want to engage with people quickly and easily, but still have an app-like user experience.
You shouldn't force people to download and install your app, when you can serve them up the same or similar content and experience via a mobile website. It's much easier to tap on a link and have the web app load instantly, rather than go through an app-install to get the same experience. Web apps are much faster to develop, hence much cheaper.