Facebook CPM vs CPC impressions?

[Skittled]

Possibly newbish question. Expecting flaming. Help would be great, too. Don't really need an answer, but it's a discrepancy I can't seem to explain, and WF is all about this shit.

I've been testing a few facebook ads under both CPC and CPM.

Why are CPC receiving more impressions than CPM? Does CPC have higher priority in FB's ad scheduling black box, giving it more impressions? Seems kinda backwards. Have a few explanations of my own, but all revolve around arbitary assumptions about FB, rather than anything from the stats I'm seeing:

Ad1-CPC: 85.5k impressions; 0.02%CTR
Ad1-CPM: 16.4k impressions; 0.02%CTR

Ad2-CPC: 33.1k impressions; 0.01%ctr
Ad2-CPM:16.7k impressions; 0.01%ctr

Never had to (on average) pay my max bid: always a good 5-10cents under it for CPC, and 4-5c under for CPM. All bids are at the lower end of the value recommended by FB. Happy to provide more stats if relevant.

Anyone else had similar experience or explanations?

 

[kingfish]

What were the costs comparatively. Like what was the cpm cost of the cpc run vs the cpm run.

 

[Skittled]

CPC ads: average actual cpc paid=.62 (resulting avg cpm=0.12), max bid of 0.65.
CPM ads: avg actual cpm paid =.25c (resulting cpc=$1.42), max bid of 0.30.

So the CPM ads are about twice as expensive (per thousand views, or per click, either way) as the CPC ads. 1.42 vs 0.62, or 0.25 vs 0.12.

 

[kingfish]

Hmm I think you just may have gotten unlucky at the start of your cpm ads and then fb slowed them down because they made them less money. If that were they case and you raised your bid for a while and ran them longer it would theoretically smooth out, but I could be wrong. I've never gotten the CPM ads to work anyway I think they're just an orphaned thing. Best if you want branding but not so much if you want affiliate actions.

 

[Skittled]

Interesting to see the level of noise in the system -- not that i've got a particularly good sample, I suppose.

Ultimately turned off CPM, as its the clicks I'm looking for (not technically aff stuff: am looking for survey respondents); just thought I'd give CPM a go given I'd got my hands on some free advertising credits.

Thanks for the interpretation, Kingfish!