From the words of old: Everything is worth what its purchaser will pay for it.

Today, that's the price of advertising.

Personally, I don't think we're witnessing a bubble. It would be one, if the levels would not be sustainable for those who pay the price, but AFAIK, they are sustainable. Companies still make a profit at the end of the day. There are companies that have been paying top dollar for advertising for years now. They know their business.