This just my personal opinion, but in the case of a "like" button, it relies on you having interacted with Facebook, having previously accepted their cookies.
In the case of most ad networks, no one knows who they are, no individual visits their site directly. Now in the case of Google, I don't know why they couldn't just serve ads from google.com and still get around this issue, since most people would have accepted cookies from them already.
In the case of most ad networks, no one knows who they are, no individual visits their site directly. Now in the case of Google, I don't know why they couldn't just serve ads from google.com and still get around this issue, since most people would have accepted cookies from them already.