Total VC funding in the US is on the order of $50 billion per year. If you assume 20% of that goes straight to advertising, which is unlikely, that's $10 billion. Facebook and Google sell about $100 billion a year worth of ads.
I would guess that Google and Facebook have less than 1% of their revenue concentrated in VC-funded startups. They simply don't have enough money to matter relative to the size of the broader economy. It is not 1999 -- Yahoo is not making headlines with money it got from startups whose valuation is driven by Yahoo making headlines.
People in the startup community think the startup community is co-extensive with the tech industry. AppAmaGooFaceSoft beg to differ.