The author does seem to miss the point that this is really only about brand engagement. I do not know anyone who would even consider /buying/ doritos from facebook (or online at all I suppose), but may have an interest in letting others know how much they like it. This is what a majority of product based companies are looking for in their website, so a low cost website with incredible traffic potential is an easy choice over creating a completely unique site.

Unless facebook releases a vending front, like amazon marketplace, that would allow small companies to easily sell goods through facebook itself.