The problem is that, if I see a 40-foot advertisement on a city bus, "Drink Buzz Cola", I know that the Buzz corporation has elected to spend serious dollars and the risk of public exposure on their brand. Internet advertising still has a smarmy one-on-one private feel to it:
"How do I trust this 'Buzz Cola' who have spent .01 cents for a 4 inch ad on MY facebook page?"
...people don't get a sense that the ad campaign is city-wide, nation-wide, international, whatever, and is being shown to millions of people...even when it is. And so the brand-building ("everybody's doing it!") doesn't work the same at a psychological level.
Most people don't go through this mental process. Brand awareness happens beneath the level of cognition anyway. You can show people unfamiliar brands repetitively in an unobtrusive context and then when you put them all together and deliberately show the person and ask, "how many of these have you seen before?", the answer will usually be "none". But if, before this, you ask them to rate a group of brands, including the ones they've been shown along with some truly new ones -- they will rate the shown group higher on various metrics even though they think they've never seen them before.
I'm not suggesting that there is actual ratiocination going on when people look at ads. But a large, public, showy ad argues from authority much more than a little flickering thing on a website (that only I can see).
"How do I trust this 'Buzz Cola' who have spent .01 cents for a 4 inch ad on MY facebook page?"
...people don't get a sense that the ad campaign is city-wide, nation-wide, international, whatever, and is being shown to millions of people...even when it is. And so the brand-building ("everybody's doing it!") doesn't work the same at a psychological level.