There are many metrics of channel shift, but in some ways advertising spend is among the more interesting.  In some crude way, it represents a collective judgement by a group of people with a strong incentive to make good judgements about the way attention, interest and importance are shifting.
In effect we are looking at a prediction market in progress.

The numbers tell a strong story:

In 2006 £2.016 billion was spent online by advertisers targeting the 31 million (GfK NOP) people connected to the internet. The growth has increased the internet’s share of all advertising revenues to 11.4%, up from 7.8% in 2005.

The results reveal that expenditure on the internet overtook
advertising in national newspapers, which last year recorded growth of
0.2% to £1.9 billion and a market share of 10.9%. In 2006, the internet
was just over half the size of the TV advertising market, which
experienced a fall of 4.7% to £3.9 billion.