Yes, the first sign of the apocalypse may be heralded by a popping sound. Yahoo announced today that its third quarter results dropped lower than expectations because of lower ad spending in two segments: cars and financial advertising.

Have advertisers started realizing that ad impressions and page views are a worthless metric?

After all, there is a reason why advertising evolved out of banner ads and into a pay-per-click model with the rise of Overture.

Well, its difficult to know the reasons behind the softer ad spending, but it can’t be but bad tidings for the blogosphere.

While some larger blogs and social networks have diversified (”Hiltonized?”) their revenues, far, far, far too many still rely on advertising revenues.

Much like the first bubble, internet businesses can be remarkably sensitive to shifts in ad spending if that’s all their revenues are based on.

However, UNLIKE the first bubble — I expect that many businesses will probably be in a better shape to weather it.

The first bubble was marked by incredible excess, overindulgent spending, and an inexplicable lack of business plans. Many businesses (and blogs) in this era tend to be far leaner; while VC’s still seem interested in investing in Web2.0 businesses (and probably haven’t jumped the shark), there certainly hasn’t been the level of press with regards to startups hemorrhaging cash on nonsensical trivialities.

In spite of their being in a better position to batten down the hatches, it won’t mean that we couldn’t see further losses, and financial pain on their behalf.

With the Darwinian pressure of lower ad spends, we might see internet companies — and blogs in particular — rush to diversify their revenue streams.

For example, could we see a renaissance of affiliate programs as internet companies begin to introduce them as a cost-effective measure — and more bloggers adopt them as a way to improve their bottom line?

Or, perhaps content plays will make a more concerted effort to make more advertising work for them. After all, the Web2.0 hasn’t really jumped the shark on a dog social network — if the dog social work is making 100 000 per month.