I'm echoing what others are saying: no web version means I won't bother at all. My phone has access to multiple great browsers. Building a good lightweight web app means I can access the content from anywhere without having to deal with annoyances of push notifications or (as much) creepy tracking. My favorite example of a mobile web app is currently Twitter Mobile [0]. I even like using it from my desktop. It's very…
Instagram has a web app, but I'd be curious to see how many users actually utilise it, in comparison to mobile app users. My guess is that mobile app users heavily outweigh web users. Instagram also chose to offer a restricted set of features on the web app, specifically to encourage using the mobile app.
That's not to say that a web app is unnecessary. I reckon that the number of web users for an app like Instagram is sufficient to warrant the existence and maintenance of a web app. But I think that the majority of social network consumers are not concerned either way, and to delay a release because the web app is still being developed would be a mistake.