I’m seeing interesting cases where Facebook, Google or other gorillas tweak content presentation, and then some other company’s business is directly impacted.  The gorillas have increasing control (and power) over user attention.

For example, Demand Media took a huge hit after Google’s search engine updates lowered rankings for low-quality content.  (DMD is down 80%).

Zynga enjoyed spectacular early success with some of the first social games on Facebook.  But as game updates clogged feeds, Facebook made presentation changes to improve feed quality, and Zynga suffered.  (ZNGA is down 60%).  Facebook continues to tweak how feed content is selected and presented.

More recently, Google added a “Promotions” tab to Gmail, moving most marketing emails out of the main inbox view.  That directly affected Groupon, who’s trying to rely less on the daily coupon.  (GRPN down 20% so far).

And now, Gmail added a prominent “unsubscribe link” to the top of promotional emails, which will impact email marketing performance even more.  (Unsubscribe is one of many “quick action” buttons that Gmail has been rolling out.)

It’s a never-ending battle between those who want user attention, and those who manage it.