I have run a 3-days long Facebook Pages Ad campaign and by the end of third day Facebook said:
For exactly what am I billed? Did I spent
33.59 PLN (~11 USD) for reaching
1,785 viewers (about 160 viewers / reachers per 1 PLN) or for likes (~3.5 like per 1 USD)?
Answer to this question is essential to me. I've received 39 likes in total for 3-days campaign, that costed me 11 USD. While Facebook promised me at least 50 likes per day for $5 (daily budged set by me) -- cite: "est. 50-200 likes per day". So, if I'm counted for likes, not for impressions (reach) that would mean that this Facebook Page Ad campaign ended up as a complete failure and money waste.
Edit: I used the very basic version of ad, by clicking
Promote Page and setting campaign length (set to 3 days) and daily budget (set to $5 / 16 PLN).