What if you picked up the telephone and instead of getting a dial tone you got an ad? You'd be angry, right? That's because the telephone is a medium of communication. When you are in the process of communicating, you don't want to be slowed down or sidetracked.
What if you picked up a dictionary and instead of finding a definition you found an ad? You'd be disgusted, right? Because a dictionary is a medium of information. And, once again, when you're looking for information, you don't want to be distracted or misdirected.
On the other hand, you turn on the tv every night and the first thing you see is an ad. It's probably annoying, but it doesn't make you angry. Why? Because tv is primarily a medium of entertainment, and you have grown accustomed to entertainment media carrying advertising.
The problem the web has as an advertising delivery system, is that it is a little of each. It's partly a medium of communication, partly an information medium, and to a lesser degree, an entertainment medium (remember, 99% of all video is still viewed on a TV.)
Because it is a hybrid, it has had a hard time finding its legs as an ad medium.
The currently fashionable idea that marketers can manipulate social media to serve their needs is largely a delusion. Yes, there will be isolated successes. But most web users are far too savvy to confuse honest social commentary with contrived social media "marketing." Social media is currently an effective way for companies to maintain good customers relations, and for smallish retailers to deliver sales promotion messages.
However, advertising on the web (i.e., commercial messages that create demand) will not be fully optimized until someone figures out how to put messages into a communication and information environment that are as acceptable as they are in an entertainment environment.
As far as I can tell, no one's figured it out yet.