Clothes and cityscapes. Clothes and food. Clothes and sunsets. That's what I mostly like on Instagram. So fashion brands being there is mostly a gigantic bonus for me. It turns out, I am definitely not the only one, as Instagram is quickly becoming one of the top online shopping destinations. While e-commerce is still not part of the app, the browsing - a.k.a. the virtual window shopping is. Instagram strengths are in the really careful curation of displayed items (the bar for artsy/beautiful/lifestyle photos on Instagram is pretty high) that are interspersed with the photos of friends (and food. and sunsets). More than impersonal Facebook or Twitter update blasts, meant for mass audience, Instagram photos exude a sense of craftsmanship and personality (while also being meant for wide audience).
A free advice for fashion brands: post a link to displayed products for each of your photos. Drive users to product page. Every opportunity is conversion opportunity. You'd not believe how much traffic you'll get.
Image above is a pic of Prada magazine ad, from my Instagram account.