Agreed. At a high level, its about users, engagement, and KPIs for the businesses. For instagram that's posts, follows, likes, and comments.
With Twitter, its tweets, follows, "favorites", and... @shouting, which is still a "tweet."
The problem with twitter is if I search for, say #occupywallstreet, I'm probably much less likely to bookmark it than I would "like" an instagram photo.
The difference ultimately lies in the fact that its about attracting investment - and twitter/(instagram/facebook) all share the same piece of that pie. So when you're posting KPIs for your business - they better look better than someone else's even though they are different "people/personalities."
The truth is all social networks are competing for the same influx of new users - those coming of age and interacting with social networks for the first time. Twitter isn't winning the minds of existing users with their hearts - they'll be looking at engagement and retention metrics for users that have joined 6 months from now, and never even knew you could once "favorite" a tweet.