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All of the tools you mentioned can be replaced by their desktop counterparts.

I would argue against using your mobile device to create relevant content. I know people want an all-in-one toolkit that does everything great but let's go with history on this, it won't work.

A jack of all trades is good but a specialized tool will always be better.

Instagram < Camera + Filter Vine < Video camera + Editing Twitter < (Medium seems to be getting more popular, maybe 140 characters really isn't the future)

I've never heard of Smule.

Again, this is simply my opinion but I feel like some folks are too fast to do everything on their 'mobile device' (let's not call it a smartPHONE).

Any examples of mobile being great for mid-longform text/code content? Github isn't an image site, and the type of content created on it is usually (not always) longer than 140 characters.

Instagram, Vine, Twitter, and the like were not written on phones. You're just deliberately misconstruing their point.