Instagram is a fantastic way for retailers to share vivid imagery building brand and product awareness. It is not, however, a seamless way to sell product. Until Instagram releases a "buy now" button or direct link out to the retailer site, retailers will release workarounds.
Marc Jacobs is the latest to release a work around. @MarcJacobsIntl directs users to a page collecting the customer's email and handle. The customer is then instructed to like products and post #shopmj for items they are interested in purchasing. Marc Jacobs then sends the customer an email with the specific product link.
This workaround is a great way to collect email and link social handles to customers. However, this workaround suffers from the same issue of other workarounds already released. The process is still too laborious and will likely lead to only marginal improvements in conversion. Until the purchase process is made simple, social media will never become social commerce.