I spotted today that LinkedIn is currently displaying a plain text advert at the top of it’s homepage which is provided by Google AdSense, containing no sponsored links or Ads by Google notice!
2 example AdSense ads displayed on LinkedIn homepage:
It looks like LinkedIn is a Google search network partner so they will have more ad formats available to them than regular AdSense publishers, but until now AdSense ads have always been marked as sponsored links even if the Ads by Google notice isn’t always displayed. An example of another Google partner displaying ads without the Ads by Google notice is MySpace but all ads have been clearly marked as sponsored links instead.
LinkedIn do display an additional Google AdSense block on this page which is clearly marked, but it seems surprising that they would list a plain advert without marking this as an ad because they’ve always been clear about making sure their ads are differentiated from natural search listings or regular links.