I think it was also more advertisers switching to facebook and new ones giving fb ads a try first and not even considering Google ads (btw the rebranding from Google adworfs is another sign that Google is losing new advertisers to FB and yet another thing pointing to that desperation).

This ultimately affects Google's future growth and it's why you'll see Google do more such "desperate" moves like trying to become a military contractor, building its own iPhone-like phones, tracking users more aggressively, and I imagine android users will soon see os-wide ads, too.