Hmm...do Tumblr, and now Posterous, qualify as niche site builders? or maybe Shopify? From all such site builder examples out there, the more successful ones s

Hmm...do Tumblr, and now Posterous, qualify as niche site builders? or maybe Shopify? From all such site builder examples out there, the more successful ones seem to nail down the underlying problem first (publishing, e-commerce, etc), and then enhance it with customization - not the other way around.