Because Google users are considered to be on a "hunt" versus Facebook users, who are considered to be on a "hike", advertisers may choose Facebook over Google because:

$$ none of these

Facebook only charges advertising partners for ads that users actually click on versus every time ads are displayed

Facebook strategically places its customers' ads alongside other complementary ads$/$content to ensure they are not displayed next to conflicting or objectionable content

Facebook ads can reach consumers who weren't necessarily looking for the product$/$service unlike Google searches

all of these