"Facebook users" or "Google users" are way too broad for a political campaign ad to target. Party A's ad will go to too many of Party B's users and vice versa. They need to be much more carefully targeted to be effective.

The purpose of a political ad is not to convince undecided people to vote for [party]. The number of undecideds is vanishingly small, so there's no ROI there. The purpose of a political ad is to convince people who have already picked a side to actually vote vs not voting. So you need to carefully target your "go vote" message to your own team.