Facebook could, I imagine, be a powerful online store for people who don't otherwise have storefronts, but people who think I'm going to go to FB to shop online at Macy's are probably the same people who thought I'd go to Second Life to shop online at Macy's.
Businesses look at facebook and ask, "How can we use this to market more to our customers?" And the answer is to make a facebook page and get bunches of followers and make posts and blah blah blah.
Seems like none of them think to ask "what do our customers want to do on facebook and how can we add value to that experience?"
I founded a company called Snapline (http://www.getsnapline.com) that helps retailers integrate Facebook functionality into their e-commerce site. We've talked to many retailers, and most are still not sure where Facebook comes into play for their business. Small retailers want to engage and increase their Facebook audience, but it's time-consuming to make consistent feed updates and it's impact on sales is pretty murky. Large retailers seeks to gain much more by engaging their Facebook audience, but implementing features on their site that leverage Facebook data is outside their core area of expertise. I think what can go a long way in solving both problems is incorporating functionality within an e-commerce store that doesn't require a lot of manpower and improves the shopping experience.
Facebook has gone the other direction ... treating each retailer like "an island" where there isn't an easy way to at-a-glance see all of the retailers and their products in one view. Sure, food and "stuff" are different things... but I think for consumers to embrace an aggregating platform for commerce, Facebook will need to flatten the view to the customers to allow them to get a view of their buying choices in a less-segregated way than things have been presented to them until now.
The idea of having a total shopping experience hosted within the Facebook ecosystem worries me, because it places too much control in the hands of an organisation that's known to be capricious.
OTOH, I don't see a need for Facebook to necessarily be involved, since I don't think people shop with all their - just a few. Not being a social shopper though, I can't comment much more than that.