These valuations aren't necessarily just a bet on advertising as we know or imagine it. At least for FB and Google, the assets that give them power to direct attention to ads can potentially be monetized in other ways if ads stop being the most lucrative.

And conversely, if you don't own an asset that redirects a massive amount of attention, you can't launch any kind of product without getting help from someone who does (whether you call that help advertising or not).