1) Digital is taking over all ad-channels
2) Digital ads can be tracked and attributed ways that were never possible with traditional media
3) It turns out most non-targeted ads aren't actually that effective, and aggregate ad spend is falling.
FB ads are effective and high value because of tracking. If tracking goes away, FB ad revenue will fall to that of banner advertising at roughly 1/100th to 1/10000th the price per impression that FB currently receives.
Such a change would effectively force a rethink of the entire consumer technology business, and see FB/GOOG re-structure to some form of subscription revenue or shutdown large feature sets and product offerings which would no longer be tenable.