Why do celebs promote products, but not social media sites?

Companies have been using celebrity endorsements to promote products and services almost since the advent of advertising.  Today celebrities are promoting everything from skin care products to automobiles.

However, while celebrities seem very willing to promote products, shouldn't the manufacturers also be requesting that celebrities  promote their social media channels as well?  If social media sites are being used to develop and cultivate relationships with customers and prospects, wouldn't it make sense to have the celebrities who are endorsing the products communicating directly with the product's users? 

Why aren't Katy Perry, Jessica Simpson or Justin Bieber posting a few comments on the Proactiv Facebook page, or writing guest blogs about skin care and how readers may benefit from Proactiv.  Sure, there are short video snippets from the celebrities on the company's website and Facebook page, similar to what you see in the Proactiv TV ads, but those videos aren't interactive. 

If Katy Perry were guest blogging on the Proactive site, can you imagine how many new visitors it would draw to the site and how many new fans it would generate?  If Justin Bieber commented even once a month on the Proactive Facebook site, how many customers would return regularly hoping for an opportunity to read his comments and seek feedback?  Talk about a strong endorsement, customer experience and relationship building opportunity?!  Why doesn't Kate Walsh blog about the luxury of her "Cadillac driving experience"?  Wouldn't that be a nice supportive effort to what most customers know are "paid" endorsement ads?

I'm not sure why more companies haven't already integrated these efforts into their celebrity endorsement deals, but believe me, its only a matter of time.  Digital cross promotion between the company and the celebrity endorser seems like a logical next step in the evolution of social media as a marketing channel.