While a binary format would be better over the wire, once it's put into memory the format really doesn't matter. In fact I would argue that the decision to use JSON versus XML versus a binary format is entirely immaterial as it'll get cached once and likely never downloaded again (essentially). At least as far as Facebook's usage is concerned (they load these 6 once and never again).

Also I think you have to pick something human readable here. One of the great things about SVG is that it's in XML so you can read it, tweak it, etc. It's super handy when you want to make small changes that only affect the image. So I see using JSON as pretty handy.

Don't forget there are many types of formats out there that have 2 types: a binary and human readable type. One used typically during debugging and one used for optimal over-the-wire transport. I don't see why this couldn't be possible at a later time.

Edit: I don't understand the downvotes. Over-optimizing is a thing. For a one time download I don't see the handfuls of milliseconds you're saving mattering. In either case of receiving a binary or JSON you will store it in an optimal format on the device...