Your "three main reasons" part is spot on. We are seeing an increase in the use of Cost Per Action programs as a way to be truly measurable since advertisers only pay for the action they desire. So as you say the trend is away from the "big" media outlets toward measurable, actionable, online advertising.

It is not advertising that is suffering, since only the major players are exposed, the perception is that there is a decline. Coincidentally there is an economic recession which paints the doom and gloom for all industries. Online Advertising is on a major growth curve.

The appearance of an advertising budget decline, is actually the expansion of advertising space. The number of places businesses can advertise now is growing daily. This causes the advertising to be spread thin giving the appearance of the decline.

There are many new ad networks now experiencing rapid growth compared to the rate of growth of the existing well known networks. This is a normal occurrence during a major growth trend in any industry. Take the auto industry for example during the 1940s and 1950s. They had more brands of auto makers than we can even remember. The same growth is occurring with online advertising.